In Medieval Mode the Spy keeps two of his three most important mechanics -- the Disguise Kit and the Backstab, while the third, the Sapper, is just no longer needed as there are no buildings.
On the other hand, the most important ways of Spychecking, namely the "bite the bullet method" and the Flamethrower are no longer here, so Spychecking is very hard.
Moreover, even if you suspect someone being a Spy or even know that it's a Spy, you can still do practically nothing until you get in melee range. And then you are actually very likely to lose the engagement, because most Spies playing Medieval are very skilled at circling you and langing the deadly Backstab. Going thirdperson works, but in a limited way: circling you is still possible, though not as easily, and your ranged aiming becomes weird, so Snipers and crossbow Medics are greatly endangered.
Against two Spies, you are most likely damned -- one of them will get in a backstabbing position. If half the enemy team are Spies, it's possibly better to just leave the server and wait for this to end.
In a normal game, a suspected Spy becomes a dead Spy very soon, or at least loses time to retreat.
How do I actually counter the Spy in Medieval Mode?