In view cone snippet

Snippet

bool CMyEnt::InViewCone( const Vector &vecSpot, float flTolerance )
{
    Vector los = ( vecSpot - GetAbsOrigin() );
    los.z = 0;
    VectorNormalize( los );
    Vector facingDir;
    AngleVectors( GetAbsAngles(), &facingDir );

    float flDot = DotProduct( los, facingDir );

    if ( flDot > flTolerance )
	return true;
    return false;
}

Extra

To pass in an angle of degrees, you must convert it.

//90 Degrees
flTolerance = cos( 90 ); 

bool bInCone = InViewCone( vOrigin, flTolerance );