I've seen on a lot of Teammatch server

invincible guys (O_o)
I can't kill him with a knife and he can with a extreme simplicity

kill 3 guys before dies ...

and on a Infiltration server
there a lot of real invincible guys ...

grenade ... he don't die
knife ... don't die
Minimi with accuracy 99% ... it's a veritable wall
sniping (I hate sniping but it was nécessary) ... need 2 shoot to die

stupid auto-balance ? or cheating ?

Its LAG.

Maybe you're lagging or something, or it could be a vest.

How can you tell if someone's hacking?

Read it.