There is in fact a way to prove it, at least in some cases. If a player gets hit and blinks, but does not display the hurt animation and doesn't seem to be affected in movement by the bullets, then you can usually assume they have fake health and will get a 3hk.

Anyway, a solution for the problem of fake health and 3hks would be for BlurredD to add a new kind of packet, the health packet, which would be broadcasted every few seconds (in addition to the usual hurt/heal packets) and would update the local health bar of all players. Fake health would only occur until the player successfully receives the health packet.