The easiest way is to make a circle with your arm.
Like this guy talks about. Tight everything to isolate your shoulder. If it’s difficult or tight or painful, make a smaller circle.
If you do it near the wall, with the wall to guide you, you get awesome feedback. Close to wall = bigger arm circle, greater mobility. Far from wall, less mobility and easier. Standing = easier. Kneeling = harder. Where ever the motion is tight or difficult, you can feel what’s “off” and holding you back. Like a tight pec, bicep or front delt. Or your shoulder shrugging up or rounding forward.
Then you just put your hand against the wall, stay in that spot and feel out what’s going on. Stretch what’s tight, squeeze what’s not working. If that doesn’t work, you’ll know what’s messed up and what to really, really focus on.