How to tell if there is a spy on the head

As a spy, you're best off spending your time working to assassinate the opposing medic. Since the other team has only one medic, it is a huge advantage to your team if you can kill (or badly hurt) the other medic, even at the expense of your own life.

Since this is a competition-level play, you can expect full communication from the other team. You will very rarely fool them into thinking you're on their team. They all should know where everybody is. (Whether or not they really do is another matter). The simplest way for you to be effective is to find a nice sport to Cloak and Dagger, wait for the enemy medic to run by, and revolver or ambassador them from a distance.

Mix up your spots, be unpredictable, and stay hidden. You may also want to spend some time hounding the enemy sniper, depending on his skill level. I've seen highlander teams completely fall apart from a skilled enemy sniper. On the off chance he's razorbacking, the ambassador will be the weapon you want to go with. Mix your disguises, but definitely use the sniper disguise - that class matches closest with this play-style.

There are a number of things that will let you know immediately if an ally is a Spy or not:

• You cannot move through enemy Spies.
• You can attack enemy Spies with melee weapons.
• Friendly spies without a disguise are not enemy spies. Enemy spies disguised as spies, however, always appear disguised.
• Most weapons will not pass through enemy Spies, while they will pass through allies. An exception to this rule is the Medic's Crusader's Crossbow, which does stick in allies.
• Teammates don't take knockback from your weapons. If they do, they're a spy.
• Watch for allies moving the wrong speed. In particular, for Scouts, Medics, low-health Escape Plan Soldiers, and Gloves of Running Urgently Heavies who move too slowly.
• Enemy Spies cannot enter your spawn room. Team-specific doors will also not open for them. Team-specific doors include all spawn doors and the secondary spawn door between RED spawn and point B on pl_badwater.
• Enemy Spies cannot shoot or stab without breaking the disguise. This does not apply to the Electro-Sapper, but that only works on Engineer buildings.
• Enemy Spies that are disguised as Spies cannot fake holding the Electro-sapper or Disguise Kit.
• Spies burn with their team color (redish-orange for RED, bluish-orange for BLU).
• Spies can disguise equipping weapons that mark you for death, but doing so won't mark them for death. These includes the Gloves of Running Urgently and the Equalizer.

In addition, certain classes have weapons that help them identify Spies:

Scout

• Mad Milk - Covers an enemy Spy in milk, making him drip white. This lasts through cloak.
• Fan O' War - Hitting an enemy Spy with the Fan O' War makes a white skull appear over their head. This lasts through cloak.
• Sandman - Hitting an enemy Spy with a baseball at medium range or longer stuns him and the word BONK appears over his head.

Soldier

• Black Box - You gain HP when you hit an enemy. This includes disguised Spies.
• Buff Banner - You gain rage when you hit an enemy. This includes disguised Spies.

Pyro

• Anything that will set someone on fire. This includes Flamethrower, Backburner, Degreaser, Flare Gun, Detonator, and Sharpened Volcano Fragment. This lasts through cloak, except the Dead Ringer, or if the spy-cicle extinguishes them.
• If you see a teammate on fire, try to airblast them. If they're really teammates, you will extinguish their fire and earn one bonus point. If they never really were on your side, they will be knocked back.

Demoman

• Ullapool Caber - Hitting an enemy Spy will make it explode, but it also deals you damage.

Engineer

• Southern Hospitality - Hitting an enemy Spy will cause them to bleed. This lasts through cloak (but again not the Dead Ringer).

Medic

• Blutsauger - You gain HP when you hit an enemy. This includes disguised Spies.

Please note that the Übersaw +25% uber effect does not trigger on disguised enemy Spies

Sniper

• Jarate - Throwing this at a Spy will immediately cover him with it, making him drip yellow. This lasts through cloak.
• Sniper Rifle - A headshot with the Sniper Rifle will kill a Spy in one shot. Keep in mind that the hitboxes for each class are different. A fully charged bodyshot will kill a Spy in one hit.
• Huntsman - Any shot with the Huntsman will leave an arrow sticking out of the Spy. A headshot will kill a Spy in one hit.
• Sydney Sleeper (charged shot) - Same effect as Jarate. A fully charged Bodyshot will kill a Spy in one hit.
• Tribalman's Shiv - Hitting an enemy Spy will cause them to bleed. This lasts through cloak, except the Dead Ringer

Spy

• Any of your knives (Knife, Your Eternal Reward, Conniver's Kunai) will backstab a disguised enemy Spy. This includes bringing up the "can backstab" animation.

Last Note

Spies can avoid death if they have the Dead Ringer out. Keep that in mind and listen for the telltale zapping decloak sound.