One check is to hold the elbow on the outside (away from the body) with the elbow bent a bit less than 90 degrees, the push on the side of the hand pushing the hand away from the body. To get a better description google valgus stress test. That will usually cause pain if there's a UCL problem, but other things can mimic a UCL problem.
It sounds like you did the right thing by pulling him and sending him to the doctor.