When friendly fire is off, you can shoot at suspected spies or use allied scouts/spies (in Team Fortress Classic) freely. If friendly fire is on, though, you can still discover Spies using the same methods, but proper Spy identification becomes more important. Many things can give away a careless Spy, such as carrying a weapon inappropriate for how the class it's disguised as is used, or openly exhibiting seemingly self-destructive behavior. The best way to discover a spy is using a Pyro's flamethrower, as its flames can flush out a Spy out of a hiding spot.
- If an Engineer is carrying a shotgun, you might want to check him.
- The same goes for Medics and the Syringe Gun.
- The same goes for Spies and the Revolver.
- Sometimes a Spy can disguise as a class your team don't have and might be oblivious enough to not notice their disguise is holding the wrong weapon (such as Engineer with the Flamethrower, Medic with the Rocket Launcher, etc...).
- Watch for players who get a little too close to a Sniper without any logical reason for being there.
- Engineers should always be doing something. (Building or defense work). If you spot one who's just hanging around, you may want to "encourage" them to get back to work.
- If someone shows up for "healing" and you don't trust their motives, feel free to deliver a Bonesaw. Once you really do hit a Spy, your friends will forgive you.
- Watch people who use teleporters; the ground under their feet will glow their team color.
- You really shouldn't need a guide to tell you this, but if you see a teammate with the same name as you have, kill them.
- The wrong class in a strange place, such as a lone Medic at a sentry nest with a engineer
- Players running away from friendly Pyros, or avoiding friendly projectiles like stickybombs or grenades.
- A sniper that won't scope ever and will not shoot, but only "scopes" when crouching as when spies disguise as a sniper with a primary that is not the Huntsman, they appear to be scoping but cannot fake a laser dot or firing.
- Look around for lone Heavies in inconspicuous areas. (As opposed to a Heavy who is normally out there among his allies in open areas)
- If you run into a disguised Spy, you'll block each others movement.
- Remember that Spies can turn invisible using their Invis Watch; fire at walls on occasion to make sure that nobody's hiding there. (Don't just hit the corners, either.)
- If you bump into an invisible Spy, you'll be able to see their outline.
- Never underestimate the power of the Dead Ringer. With this, Spies can easily fake their death, which gives them an opportunity to deliver a lethal backstab on your team.
- If you're an Engineer and need to remove a Sapper from a building, make sure the offending Spy isn't waiting to backstab you while you take it off, or will quickly put another sapper on it just as you remove it.
- Remember that you might run into a Spy...disguised as a Spy.
- Watch for slow Scouts and Medics, for medics this is harder since they are only 7% faster but is noticeable.
- Watch for Medics with a 0% ubercharge, as medics shouldn't have 0% since they heal often.
- Spies with no disguise masks are not enemy spies, as disguising as a enemy spy will always give them a random disguise mask.
- Overheal that is the wrong team color is a sure indicator of a spy.
- Spies cannot inherit the double jump of a scout, along with it's speed so look for a "scout" that could easily have double jumped but didn't.
- Spies cannot fake holding the disguise kit, so if you see a friendly colored spy holding one it is not a spy whether he has a mask or not.
- The Conniver's Kunai allow free overheal for the spy. Look for suspicious classes on oddly high overheal, as they are likely spies, such as a engineer having overheal behind "his" sentry.
- The Your Eternal Reward lets spies disguise instantly as the person they backstab, but cannot disguise normally. Try to take them out quickly before they get a chance to disguise, as they can even backstab in a sentry's range and not be targeted, allowing for severe sentry crippling methods.
- If an enemy spy is disguised as a class friendly to you, and you see a outline of a semi-invisible "friendly" teammate, they are a spy since only the spy can cloak and you can always see a outline of a cloaked friendly spy.
- A disguised Spy always have their mask on disguising as a random class. If a player is seen with their hats/cosmetics (given that they're a Spy) they're not an enemy spy. If you see a "teammate" with a mask on, you would probably want to check him.
- Only an Engineer can stand on his/her own building, any other friendly class (even other Engineers) would simply walk past the building. If you see any teammate standing on top of a building or even the Enginneer (one that 'seems' to be the builder), don't hesitate to dig their grave.