See this answer, except use Z instead of X.

Calculate the angel between your forward vector and the enemy position. If absolute value is bigger than a certain threshold (eg. 90 degrees) it's behind you. To do that there a some very convenient functions:

Vector3 directionToTarget = transform.position - enemy.position;
float angel = Vector3.Angel(transform.forward, directionToTarget);
if (Mathf.Abs(angel) > 90)
    Debug.Log("target is behind me");

Not tested but should work.

I know it's been a month since any comments were left on this, but posting as the link in the answer no longer works. So regarding spree 1's code above, for complete 360 degree detection, I think it should be;

    if (Mathf.Abs(angel) > 90 && Mathf.Abs(angel) < 270)
        Debug.Log("target is in front of me"); 

    if (Mathf.Abs(angel) < 90 || Mathf.Abs(angel) > 270)
        Debug.Log("target is behind me");