The inexpensive microwave detectors you can buy in the store aren't reliable. To test for leakage, try holding a fluorescent light bulb in front of the oven and moving it around while the oven is on. If the bulb lights up or flickers, you've got a problem. Don't use the oven until it's tested by a professional and repaired or replaced. There is also a question as to whether microwaving alters protein chemistry in ways that might be harmful.
Some evidence suggests a problem.
So I'd only use microwaving for rapid heating or defrosting - not long cooking. Don't put a casserole in there and leave it there for 45 minutes or longer. Don't roast a chicken. A more immediate danger involves the container or wrapping you're using for your dinner. Microwaving will drive plastic molecules from plastic wrap or containers into your food.