The map was Well, our spy was disguised as a medic, and was “helping” the enemy to hold the top point 3 ramp.

One of the enemy’s players then said in chat: “Goddammit medic! Heal me already! Stop standing around like a fool holding the syringe gun!”.
I saw the whole thing, spectating the spy while waiting for spawning, and burst out laughing - That player actually listed the entire method, of how to know when a medic is not really a team mate, and more generally how to spot a spy.
A medic:
- Will always have an Ubercharge percentage under his name.
- Will usually hold his medigun, and not syringe gun/bonesaw.
- Will heal nearby teammates.
- Will stay out of the line of fire.
A spy disguised:
- Will hold the default weapon of each class:
- Scout: Scattergun
- Soldier: Rocket Launcher
- Pyro: Flamethrower
- Demoman: Pipe Launcher
- Heavy: Minigun
- Engineer: Shotgun
- Medic: Syringe Gun
- Sniper: Sniper Rifle
- Spy: Pistol
- Will not be shooting.
- Will not be doing what he’s supposed to be doing in the game (smart players will make you believe they are in fact helping your team, but can still be spotted by various discrepancies, such as the capture multiplier not counting them as helping the cap when stepping on a point).
- Will block your path, unlike teammates which you can partially clip through.
- Will do his best to stay away from Pyros.
- Will call for a medic, even when his health is high, or even full.
- If disguised as a scout, will be noticeably slower.
- Will not have an Ubercharge percentage under his name, when disguised as a medic.
A cloaked spy:
- Will shimmer in his team’s color for a brief moment when either hit by a weapon, or bumped into by a member of the opposing team.
- Will leave a visible trail if he has recently used a teleporter.
- Will have visible droplets of water falling off if he has very recently been underwater.
- His health will not fill while being healed by a medic (Thanks BaronHVG!)
However, the best way to reveal a spy is to simply trust your intuition. After enough time playing, you can gain a “sense” of when someone is acting a bit suspiciously. The best way to then confirm your suspicion is to try to walk through the suspicious player. As stated above, if he is a spy you won’t be able to partially clip through him. If this is the case, you are more than welcome to dispose of him, preferably with your melee weapon as to add insult to injury.
