How do i check what a player can see in their sight

the title speaks for itself , ply:GetEyeTrace or GetEyeTraceNoCursor cuz they didnt work

GetEyeTrace returns a table with many things. If you want to get what is the player looking at, use ply:GetEyeTrace().Entity.

More info here:

If you want to check if something is within a player line of sight, that would be a bit more complicated. Perhaps you’ll find the solution here:
http://forum.facepunch.com/showthread.php?t=1362830