I'm not too good with posting pictures, but to palpate, I'm pretty sure it's either my anterior deltoid or corocobrachialis. The pain starts right under the corocoid process and then it runs down the front of my shoulder out towards my humerus. Somtimes it seems like it radiates out to the insertion of my deltoid. It also seems to radiate down my humerus sometimes towards my elbow, kind of along the long head of my biceps.
The movement that hurts the worst/most predictably is shoulder abduction, up at the top I can just feel the pain like right under the lip of my pec/anterior delt. It doesn't hurt in the beginning of the movement, just the top half I would say.
It also hurts to be pushed into external rotation when my humerus is adducted (at my side) while my elbow in in flexion. Or if I swing my arms back into the position at the bottom of a bench press. Finally, it hurts up at the top of shoulder flexion. Again, doesn't really hurt on the way up, just when I get to the top. When it's not moving it just kind of hurts a little bit all the time.
Edit: It may be obvious, but pressing hurts too, both horizontal and vertical.