I think somethig like this will work, if not, you can try post hook.
But do not run this on <1.75 because of that bug:
http://forums.alliedmods.net/showthread.php?t=28004
Code:
new g_phit[33]
public plugin_init() {
register_forward(FM_TraceLine, "forward_trace_line")
}
public forward_trace_line(const Float:start[3], const Float:dest[3], ignore_monsters, skip_ent, ptr) {
if (skip_ent && is_user_alive(skip_ent)) {
new phit = get_tr2(ptr, TR_pHit)
if (phit != g_phit[skip_ent]) {
if (phit && is_user_alive(phit))
server_print("%d start aiming at %d", skip_ent, phit)
g_phit[skip_ent] = phit
}
}
}