Possible rotator cuff tear or impingement? how can i tell?

Rotator Cuff: In most cases it's very difficult to tell the difference between the two, however significant loss of function (i.e. Strength) is one good way to determine if the cuff is torn rather than just pinched. Even then, advanced imaging such as MRI may be required and in some cases a steroid injection may help your doctor determine the difference. Best of luck!

Best way to tell...: Is to see a doctor. A physical exam will lead to the diagnosis. This can be confirmed by mri. Thank you.