There is no official expression to get it.
You will have to do some work <img src="smileys/smiley4.gif" border="0" align="middle" />
Basically you need a global variable, name it appropriately.
I named my LayerAngle
Now make the event to handle the rotation, For this example i'll just do a mouse click.
Mouse.OnClick[Left]
->System.addTo[LayerAngle(90)]
->System.SetLayerAngle([Layer], LayerAngle)
System.CompareTwoVales(LayerAngle > 360)
-> System.SetValue(LayerAngle(0))
That should work, Let me know if you need a capx