The only external inspection possible on a master cylinder is to see if fluid is getting past the rear secondary seal and running down the booster below. That's it.
A consideration is the age of the master. If it's more than six or seven years old, it's on borrowed time.
Calipers can be checked for free-moving pistons, and leaks, as can every other connection in the system. But not the master.
So if it's old, and you believe your son, install a new master cylinder.