The cheapest way is to befriend a person who owns a microwave leak detector and then ask if you can borrow it. The next cheapest way is to buy one yourself or with a group of people. Prices for electronic leak detectors start at under US$40, so they are quite inexpensive. Generally speaking, a clean microwave will not leak sufficient radiation to do any harm. Still, it is a good idea not to stand directly in front, peering through the door. For safety, you should always stand to the side while it is running. Most leakage occurs around the door seal, which should be cleaned regularly to ensure the door closes tightly and the seal works properly. A microwave with a loose door will leak. If your door is loose, repair it or discard the unit. Some older models had substandard seals, but most of these disappeared from the market years ago. They may still turn up in yard sales, though. It is always better to keep consumer products running as long as possible to keep them out of the landfill. Buy quality consumer products that will work for a long time, rather than cheap junk which dies in a year or two. It's cheaper in the long run all around. However, a microwave should always be discarded if the door seal cannot be made safe.