[Ahmadd]

How to detect player when s/he's desyned?

[Iguman]

You could try changing player's world and if it doesn't change on your screen, he's desynced.

[Maxandmov]

Try to manipulate his character's state, like TP him or something of sorts.

[GangstaSunny.]

https://sampwiki.blast.hk/wiki/GetPlayerNetworkStats
As far as i know send/recived bytes wont go up if timed out.