The first sign is a crankcase full of gasoline, but if you are having carburator problems it can be difficult to tell which one is the problem. The diaphragm is open to the crankcase on the back side. One way to tell would be to do a leakdown test. Disconnect both lines, plug the outlet, and pump up the inlet to about 4 psi with a hand pump. It should hold the pressure for a long time, otherwise it leaks.
Another way to tell would be to run the engine off another pump or gravity feed it and see if you still get gas in the oil.
