At least for starting pitchers, it can be hard to tell if there's a shoulder injury or just extra after-start swelling. Sometimes it is perfectly, like when there's a really sharp pain. Other times, it's shrug off as usual swelling that will clear up in a day or two. After a couple of days, it may still be sore a bit but pitching feels fine once they warm up. Rinse and repeat X number of starts until one day the sharp pain comes and doesn't go away. That's when they usually tell someone because at that point the usually and very visually can't really pitch.
