[ConnorMcLeod]

You could use ExecuteHam(Ham_FVecVisible, this, const Float: origin[3]), then you wouldn't have to retrieve player eyes position.

BOOL CBaseEntity :: FVisible ( const Vector &vecOrigin )
{
	TraceResult tr;
	Vector		vecLookerOrigin;
	
	vecLookerOrigin = EyePosition();//look through the caller's 'eyes'

	UTIL_TraceLine(vecLookerOrigin, vecOrigin, ignore_monsters, ignore_glass, ENT(pev)/*pentIgnore*/, &tr);
	
	if (tr.flFraction != 1.0)
	{
		return FALSE;// Line of sight is not established
	}
	else
	{
		return TRUE;// line of sight is valid.
	}
}