Mark has a post on this:
How to Maintain Shoulder Mobility and Scapular Stability | Mark's Daily Apple
WebMd is a good resource for this too. Go to the symptom checker and they explain it.
I'm recovering from something like this at the moment, from bad posture at work. Hold your arm out straight front at shoulder height, now turn your arm so your thumb is pointing down. Do you feel a sting deep inside? Not a surface soreness -- deeper mild pain. Do the same holding your arm at shoulder height out to the side. Test around for flexibility. In another thread, Dave Mayo here at MDA suggested that you put your hand on your opposite shoulder and lift your elbow up to your forehead. Feel any deep stings? How about when you make a sharp turn while you're driving? These are all tests for tendon inflammation.
STOP using the arm! Even if you don't feel it you may be injuring it more. Sleep on your back if you can (any other position can stretch the injury, I found this out). If it doesn't improve in a week or two, get it checked. Rest and ice/heat will also help. If your family can stand the smell, Ben-Gay is good too.
If your arm hurts all the way down to your wrists, or is numb or tingly, that's a bad sign that something might be torn. Get it checked immediately.
5'0" female, 45 years old. Started Primal October 31, 2011, at a skinny fat 111.5 lbs. Low weight: 99.5 lb on a fast. Gained back to 115(!) on SAD chocolate, potato chips, and stress. Currently 111.