Membership Token

Oh Crap, you've been cut off from your friends, perhaps while performing a Last Stand, and have been captured by the enemy. You escape, but how can you prove you're a good guy when the bad guys have been trying to infiltrate your organization, perhaps using stolen or faked uniforms?

Alternately, what happens if, once you make your escape, you need to prove your identity to the local chapter of your friendly neighborhood La Résistance or your own backing organization, but due to the nature of your business, you don't exactly carry company ID cards on you?

And of course, sometimes you're in a club that likes to be have something akin to a Secret Handshake, in which case this can become Serious Business indeed.

Why, then you need a Membership Token! This token is something discreet, yet distinctive, that you use to identify yourself to your fellows but isn't readily familiar to outsiders. It might be some rare or specific item (possibly even some sort of MacGuffin) that not just anyone would have, or something more common but otherwise difficult to get, such as one of a relatively small set of coins with a distinct design.

See also Calling Card and My Card. Can overlap with Iconic Item for some of the more famous examples.

Anime

 * In Revolutionary Girl Utena, the ring with the rose crest instantly identifies Utena as a Rose Duelist to her fellow Duelists, and allows entry to the Secret Forest. But Utena herself no knowledge of the purpose of the ring; she had been carrying it for years as a memento of her prince, who said it would lead her back to him. Though the the true meaning of the ring is... complicated.

Film

 * The "Nine Pieces of Whatever We Have in Our Pockets" from Pirates of the Caribbean: At World's End.
 * And from the Curse of the Black Pearl, the gold coins from the cursed treasure. Elizabeth Swan unwittingly convinced the Black Pearl's crew that she was Bootstrap Bill's daughter because she had his coin, along with claiming Will's last name (in order to prevent them from realizing she was the Governor's daughter).
 * It's subtle, but in Iron Man, both Rhodey and Tony Stark wear the "Brass Rat", the class ring of MIT, though no particular attention is drawn to it.
 * The guys in Chris Cutter's curling team in Men With Brooms all have beaver tattoos
 * In the sadly unappreciated Wing Commander movie, Taggart is able to earn the trust of the Tiger 's Claw's captain because he has
 * From the same film, the Pilgrims' crosses.
 * The a Team's Army Ranger tattoos work like this at the beginning of The Movie.
 * The ankh necklaces in Logan's Run are used by runners to identify one another.

Literature

 * E.E. "Doc" Smith's Lensmen had the Lens, which glows with light when its owner touches it.
 * Randall Garrett's Lord Darcy stories had King's Messengers, who had begemmed badges that glowed red for their owners. This was a Shout Out to a Lensman's Lens.
 * In one The Saint story Simon Templar finds a dead body with a silver greyhound badge on it - the deceased was a King's Messenger.
 * In Don Pendleton's The Executioner stories, top Mafia enforcers used aces (playing cards) to identify themselves to other Mafia members.
 * In Harry Potter and the Order of the Phoenix, Hermione creates special enchanted gold coins that look like Galleons for Harry to use to communicate when the Dumbledore's Army would meet to practice the forbidden Defense Against the Dark Arts techniques. They come again in DeathlyHallows to call for aid to the members.

Live Action TV

 * On Babylon 5, members of the Anla-shok, or as they are known to the humans, The Rangers, wear a distinct brooch showing a Minbari and a Human on either side of a large green stone. Earlier in the series, when the Rangers were a covert intelligence-gathering organization, this was used to allow them to discretely identify each other in public Once they become an overt military organization, and even later again become a form of Space Police, it is used as insignia on their uniforms instead.

Tabletop RPG

 * Call Of Cthulhu supplement The Asylum and Other Tales, adventure "The Asylum". Freygan's cultists have recognition symbols: small stones with thongs for wearing around the neck. They are rarely used since all nearby cult members know each other by sight.
 * Dungeons And Dragons. AD&D 2 ed. has some spells working via designated tokens -- including "pass-cards" for warding spells and focus for mass mirror image.
 * In Forgotten Realms the Harpers have a symbol they use to identify each other: a small silver harp. After the Moonstars did split off, they use their own variant of the same. Cormyr has Purple Dragons' rings and commanders' rings that (aside of minor magics) allow access to restricted and magically protected areas, like the armory in Purple Dragons' barracks. Of course, these items are badges of service and possession of one by someone not entitled to this is a crime.
 * Al-Quadim has a lot of secret societies with their own ways of identification; and appropriate magic, like a spell sending a distress call to the nearby fellow member, that works only on someone having a proper token.

Video Games

 * A minor Gotta Catch Em All sidequest in Mass Effect had you searching the galaxy for medallions belonging to members of the League Of One, a Salarian precurser to the Spectres.

Web Comics

 * Lackadaisy Cats has its club pendant, often hidden under a layer of clothes, which is used to get into the speakeasy.

Western Animation

 * Avatar: The Last Airbender combines this with Secret Handshake: To identify members, the Order of the White Lotus uses a special set of moves on a mah-jong style game. To play said special move, one must be in possession of a White Lotus Tile, and the tiles seem to be used as calling cards for the organization.

Real Life

 * Challenge Coins date back to a squadron of American pilots during World War One. One of the pilots, fairly well-off, decided to have a set of coins minted with their squadron's name on it for his squadron mates to have as keepsakes. One of these pilots was shot down, captured, and deprived of his military identification by German troops. An artillery barrage caused enough confusion for him to escape to the Allied lines, where he was able to prove his not being a German spy only by presenting his squadron's coin to the Allied troops. Ever since then, failure to present your coin when challenged results in a severe punishment. That is to say, you buying beers for everybody who has their coins. Failing a "Coin Check" while in a large drinking establishment can become...expensive. You can be challenged at any time. Even when you're on the toilet or in the shower. It's sacred tradition.
 * In the US Air Force, airmen are issued their first challenge coin, called an Airman's Coin, when they graduate Basic Training. Effectively, this means nobody in the Air Force has any excuse at all not to have a coin on them. Those who argue that they don't respond to coin checks because the coin is too important to them are Completely Missing the Point (again, Serious Business.)
 * It is very common for specific groups within the military (ie: a particular Squadron or Company, or a group of soldiers who worked together on some endeavor) to mint coins specific to them, or to commemorate special events.
 * The rules vary significantly from one unit to another. In some cases, nearly any coin will do. In other cases, it's not enough to have a coin on you. You have to have the senior coin (issued by someone of high-rank, such as a general or an admiral).
 * Not everyone is a supporter or participant in this tradition. Many see it as a way for cheapskates to get unsuspecting marks to buy drinks. This is especially true when challenging troops new to a unit who have no way of knowing the rules. Some units avoid this by having the rules specifically exempt those who have not yet been given their unit-specific coin or had the rules explained to them.
 * Graduates from many colleges use their class rings for this trope. People who draw attention to themselves by tapping the ring on their finger against whatever surface is available (tabletops, counters, etc.) are known as "Ring Knockers".
 * Many engineers in the United States and Canada wear cast iron or stainless steel rings on the little finger of their writing hand. This way, whenever they use a pen or pencil, the ring will drag across the writing surface and/or bite into the skin slightly (The Iron Ring used in Canada is faceted rather than completely round, unlike the Engineer's Ring used in the US), reminding them of their responsibility as engineers. Of course, the ring also helps fellow engineers identify each other when out in public.