How do I know if it is tendinitis, arthritis, or other in the shoulder?

Sx are similar! You need a clinicians expertise to differentiate arthritis, tendinitis, and bursitis of the shoulder.If they co-exist, inflammation is likely.If you are young, arthritis of the shoulder would be inflammatory, most likely. If all three exist, arthritis most likely.However, all three can be degenerative, caused by injury, overuse, spurs (impingement syndrome, etc. An exam and x-rays, and a few labs!