We Need to Get Proof

This is a common plot device often used to kickstart a story, or get it into high gear.

Basically, the heroes discover that some bad guy is up to no good. The problem is that no-one will believe the hero, for whatever reason. Maybe because the hero is a kid, because the bad guys are respected authority figures, or because their scheme sounds outlandish. Either way, the heroes need proof of what the bad guys are doing in order to get anyone to believe them, and the ensuing adventure becomes essentially about that. Of course, this plot isn't quite able to catch up in this day and age of hand-held phone recorders/cameras/camcorders in cell phones; because that would get in the way of the drama of trying to acquire evidence. Often they'll come up with different ways of recording information that's complicated enough for the plot.

This is also a way to get otherwise reluctant heroes involved in the plot. If for no other reason than the fact that since they can't convince anyone else, it's all up to them to solve the problem.

This can also affect the genre, and is the catalyst of many a Amateur Sleuth and Snooping Little Kid story.

Anime

 * The main obstacle for L of Death Note is not in deducing Kira's identity, but in finding proof beyond a shadow of a doubt to support his deduction.
 * Not unreasonable, given that Kira is using a magic killer notebook to commit his crimes. Without proof, who would believe that?
 * What Tenma of Monster occasionally tries to do about Johann. The rest of the time he's attempting to kill him.

Literature

 * My Teacher Is an Alien starts off with Susan Simmons finding out that, well, look at the title. She knows full well no-one will believe her, and her efforts to try to get evidence are what set the story in motion.
 * A Spy in The Neighborhood is about three boys who think a certain neighbor may be a spy. They want to get the cops on her case, but in order to do that, they need to find proof of some sort, so they basically spy on her.
 * Jennifer the Jerk Is Missing starts out this way, as Malcolm needs her babysitter, Amy, to believe that he did indeed witness a kidnapping. They then try to prove it to the adults, which doesn't work.
 * Animorphs starts off with the kids trying to expose the Yeerks' silent invasion to the world, in hopes that once the world knows what's going on, they'll be able to fight back. This plan becomes less significant as the series progresses, in favor of simple guerrilla war, but ultimately succeeds near the end.
 * This happens in a few of the Alvin Fernald books by Clifford B. Hicks
 * A major plot thread in Harry Potter involves the titular character being certain that Goldfish Poop Gang leader Draco Malfoy is up to something, and trying (without success) to prove it. Of course, this is in large part due to the fact that the primary responsible authority figures

Live Action TV

 * In Babylon Five, several episodes involved a search for evidence to prove that.
 * The Fugitive combines this with Stern Chase, as a wrongfully-accused man flees from the law as he tries to find the evidence to prove who the real killer was.
 * The X-Files. If Scully had a catch phrase, "We need scientific proof!" would be it.
 * Eerie Indiana
 * On Saving Grace, Rhetta is a forensic investigator and does all kinds of tests on any items that Earl gives Grace to prove his angelic existence. Boy, does she find proof.
 * Subverted in Star Trek Deep Space Nine. When Sisko sets out to find proof that the Dominion is going to attack the Romulans, resident spy-in-exile Garak tells him the best way to find it will be to manufacture it themselves.
 * Happens a lot in White Collar. The criminals are often known right away; the trick is gathering enough evidence to allow the FBI to do anything about it.
 * A common occurance on CSI and its spinoffs. Often occurs on cases where the evidence isn't yielding as many leads as the CSIs need to make the case.

Tabletop Games

 * Hunter: The Vigil's Network Zero has proof of nearly goddamn anything. The problem is getting people to take it seriously.

Video Games

 * The early plot of Mass Effect revolves around the protagonist uncovering evidence to prove that the villain is actually the villain to the not-so-omniscient council of very visible politicians.
 * Your first mission in Phantasy Star 2 is to recover a recorder from the Bio Systems laboratory in order to find out why the lab started making monsters. This is but the first step in proving that the Mother Brain is not as benevolent as she appears to be...
 * In Baldurs Gate II you're required to find overwhelming evidence of the younger Roenall's wrongdoings, as all complaints about him have to go through...him. So you need something very convincing to convince his superior to break protocol and get rid of him.
 * In the NES Dick Tracy game, as Dick is a by-the-book cop, you have to follow the trail and find a LOT of evidence before you can arrest any of the big villains, even though you can confront them directly at any time. Arresting them prematurely earns you a stern talking to from your boss. (Much to the chagrin of The Angry Video Game Nerd.)
 * Kingdom Hearts: In Wonderland, Sora and company must clear Alice's name regarding an assassination attempt at the Queen and find evidence against the Heartless.
 * In Beyond Good and Evil, this is basically the entire plot: to prove the Alpha Sections are secretly associated with the Domz they're supposed to protect against.
 * In Fallout, after saving the mayor of a little town from an assassination attempt, he asks you to gather evidence on the person (a local crime boss casino owner) he knows is behind it. You can either side with the crime boss, pretend to be an assassin available for work to make him do an Engineered Public Confession thanks to a tape recorder the mayor gave you, or just plain bugging his room.

Web Original

 * In Dept Heaven Apocrypha, the main characters involved in Nessiah's plot spent months trying to convince Asgard that . To make things worse, it looks like said character's influence is going to quash all possibility of a fair and just trial.

Western Animation

 * Zim of Invader Zim is a Devil in Plain Sight whose Paper-Thin Disguise shouldn't need any proof to expose, but the earth is largely populated by idiots so Dib is continually after "proof" (generally, a shot of Zim not in disguise) -- he's managed to get it on more than one occasion, but since Failure Is the Only Option he always loses it even if he otherwise thwarts Zim's latest Zany Scheme-but-Evil Scheme.
 * A common occurence for Candace in Phineas and Ferb, despite the guys not being evil (at least not to us). Their inventions are always in plain sight, but their sister goes through gread ordeals to actually get their mom to see it, resulting in this trope several times by using cameras et.c.