Most microwave ovens tested by the U.S. Food and Drug Administration show little to no microwave radiation leakage. While the FDA allows a microwave to leak up to 5 milliwatts of radiation per square centimeter during its lifetime, this is lower than any known harmful limit.
While many microwave testing devices are on the market, the FDA has found that many of them are inaccurate or only provide general readings rather than specific leakage numbers. Since microwave windows are shielded, in general, the only way a microwave can leak radiation is if the door's latch or hinges are damaged. The safest way to avoid any potential radiation leakage from a microwave is to step away from it while it's running.