If thumbs are pointing up, the arm is in some relative amount of external rotation.
If the thumbs are pointing backwards, your arm is in maximum external rotation (or more if it can start to point towards the ground).
You can tell by the fact that the biceps is rotated fully on top of your arm or not. You'll see if you turn your thumb more from pointed up to backward that your biceps will be more on top. That's more relativel external rotation.