If you suspect that your brother-in-law is cheating on his wife, here's an easy way to find out for sure: Have him take a DNA test. Research has shown that men will often change their behavior in order to see if they're genetically related to someone other than their current spouse. By taking this test, your brother-in-law can prove whether or not he's been unfaithful. The DNA test also gives you both peace of mind - knowing that you'll be able to get over any possible issues between you if the results come back negative.