Violence Detector

An ability (usually a power for a specific person) to recognise that violence or bloodshed has been done at a certain place, or by a certain person. This is one of the standard Psychic Powers- see "postcognition" and "psychometry" on that page. Being able to sense the history of an area or object is useful, and violent events in said history just stand out. The psychic in question may or may not be able to discern who perpetrated the violence, and against whom. They can usually determine the severity of it, especially when it was murder, and especially when the body was buried or hidden nearby. Note, however, that solving a murder by communicating with the ghost of the victim isn't the same thing.

A less fantastical way of detecting violence works by the smell of blood, and thus is a common ability for beings with powerful noses (dogs, (were)wolves) or an affinity for finding blood (vampires). This makes sense as blood has very distinctive and pervasive odour, has the tendency to seep into materials, and is extremely hard to clean up completely. However, the blood here may well be metaphorical, especially if the bloodshed happened many centuries ago or a concerted effort has been made to clean it up using the right equipment. As such, this ability often goes beyond what natural senses can explain.

Finally, there is one kind of Violence Detector which has the benefit of being absolutely real (though of course its use in media tends to be simplified), and that's Luminol, a chemical powder which has the quite neat effect of making blood glow in the dark. The applications of this are obvious, and Luminol is a favorite trick-of-the-trade in forensics shows such as CSI and Bones, as well as crime and courtroom dramas such as the Phoenix Wright games. Real Luminol works just about the same as shown, but it not quite as useful, mainly because there are other things that can cause the same glowing and so further testing is required on any bloodstains found.

This trope is of course the bane of murderers everywhere, and can be used to easily point out a guilty party where guilt is hard to place. An evildoer effectively has a Red Right Hand (in some cases literally) to a character with the proper sensitivities. Which is why limitations are often in place to keep the heroes from figuring it out right away.

When a person with this ability senses their own past wrongdoings, it can lead to an Out, Damned Spot!. Often in the repertoire of an Evil-Detecting Dog.

Note that detecting violence isn't the same as detecting a killing intent. Often said intent is indicated by a simple Death Glare, or a change in a character's Battle Aura. But when a character is psychically or by other supernatural means able to detect harmful intentions, it counts.

Examples of Violence Detector include:

Anime and Manga

The Kirin in The Twelve Kingdoms are so sensitive to blood (prolonged exposure to it can kill them) that merely being in the presence of someone who regularly commits violence (most usually a soldier) can make them feel ill.
The titualar character in Inuyasha has used his ability to smell blood many times, sometimes from miles away.
In Zettai Karen Children, this is one of the more frequently-seen uses for Shiho's Psychometry ability. She is shown, on at least on occasion, to help the police with unsolved crimes (her dad is the Chief of Police) simply by touching various objects related to murders, and then giving them a whole list of details - premeditated or not, identity of the killer, his mindset at the time, ect. She can also immediately identify a dangerous person at a touch. Unfortunately, this has left her constantly exposed to the darkest corners of the human mind since an early age, and as a result, she's frighteningly cynical and downright cold at times.
In Yu Yu Hakusho, Yuu Kaito can set up a psychic territory where no violence is permitted - his power detects violence in the sense that it recognizes violent acts and shields the target.
Mai of Ghost Hunt has periodic psychic dreams pertaining to the case her team is on at the time, usually intimating details about the spirits who have suffered in those locations.

Film

In the movie Hero, the Emperor is able to detect the title character's murderous impulses, shown to the audience as a repeated blast of wind threatening to extinguish the rows of candles separating the two.

Literature

Sgt. Angua has used her hypersensitive werewolf nose to point out suspicious figures and find corpses on many occasions. The price is that the smell of blood is overpowering, hampering her around messy murders.
The Wheel of Time gives us a kind of person called a Sniffer, who can supernaturally smell that violence has been done in a place, and how long ago. They are found only in the Borderlands, for reasons unknown. Although Sniffers are said to be about as rare/common as magic-users, only one Sniffer is encountered in the whole book series.
Anita Blake accidentally touches a neurotic psychic, who starts flipping out about all the blood on her aura.
Forensic magicians in the Lord Darcy stories can do this.

Live Action TV

As mentioned above, CSI and Bones both use Luminol as a simple way to check for blood at the scene of a crime, which can then be matched to a victim/perp via DNA testing (which, by the way, takes more than a few hours in Real Life). On the above shows, a black light is used to enhance visibility, making the use of Luminol even cooler-looking.
In the fifth season of Angel Wolfram and Hart had psychics with this capability, and Angel used them to examine a house where a relevant atrocity had taken place years ealier, on the pretext of being potential buyers.

Psychic: The walls scream with the blood of the innocent.
Realtor: I'll be in the car.

Tabletop Games

Traveller. The K'kree were vegetarian aliens with an acute sense of smell. Any humans who wanted to meet with them had to stop eating meat for several days ahead of time to avoid offending them.
The Villains and Vigilantes adventure Devil's Domain has the Xareon, a race of Plant Aliens. They refuse to accompany the party because they can sense that the PCs eat vegetable matter, which disgusts them (sort of how humans feel about I'm a Humanitarian creatures).
In Vampire: The Masquerade, committing diablerie (draining another vampire of its life force) left a stain on a vampire's soul that could be read by other vampires.

Video Games

In Oblivion, if you have committed murders, guards may greet you with:

You. I've seen your kind before. You've got blood on your hands. Keep your blade sheathed, you murdering bastard, or I'll put you down myself.

It's clear this isn't literal blood but the psychic stain of murder that gives you Infamy, even if your killings were not witnessed.

The Protoss High Templar unit in StarCraft has these Stop Poking Me lines: "Your thoughts betray you." and "I see you have an appetite for destruction."