How do you check for a radiation leak in a microwave?

The only way to check for microwave leaks is to check it with an radiofrequency radiation (RFR) probe, which are very expensive and require technical training to operate.

Unless the microwave, and especially the door, have noticeable damage you can be very confident that exposure is very negligible and that no adverse health effects will come of it. As an added precaution, it is smart to stay several feet away from the microwave while it is cooking, since radiation levels drop sharply with increased distance.

You can buy a leakage detector for $40. Please note that these detectors are not certified by any government regulatory agency and therefore the manufacturer can make any claims they want - similar to the claims seen on "supplements" sold at the drug store.