That is not really how it works. The medical staff are professionals that have been evaluating players for a long time. They generally know if a player is injured. They will check range of motion and have him do some movements to see how he reacts. They also have diagnostic tools in modern medicine (MRI, Ultra-Sound, etc..). If they have him do calf-raises with weight, he won't be able to fake not having pain. Another way they evaluate is in the weight-room. If he can't do his normal weights when lifted, then they know something is wrong.