If you have pain in your shoulder, inability to raise your shoulder over head, cracking or grinding in your shoulder when you attempt to use it, inability to participate in overhead activities. Those would be times to go in and see an orthopedic surgeon and ask some questions. Although, the need for shoulder replacement is often dependent on a combination of symptoms and X-ray findings. Both of those things would require an evaluation by an orthopedic surgeon who could get appropriate X-rays and talk to you about your symptoms.
There are several reasons someone would consider a total shoulder replacement. Osteoarthritis of the shoulder joint, fractures within the shoulder joint, and/or rotator cuff tears or deficiency, or a combination of any of those three conditions.