[header] How to know if a guy is cheating on you [title] Look for obvious evidence of another woman. [step] It may seem like a cliché, but finding a smear of lip gloss in a shade you don't wear on his shirt is a good sign that he's cheating. Smelling another woman's perfume on his hair or clothing is another warning sign. 
[substeps] If you notice a lipstick stain or strange perfume on your boyfriend once, it may just be from a friend or relative, so you shouldn't necessarily worry. However, if it happens more than once, you may have cause for concern.