There is no surefire way to tell if a girl has slept with another guy, but there are some indicators that may suggest she has. For example, if she is comfortable talking about her sexual experiences with other people, or if she has few inhibitions when it comes to sex, it's possible that she has slept with someone else. Additionally, if she frequently brings up other guys in conversation or seems more interested in them than you, it's possible that she has been intimate with someone else. Ultimately, the best way to know for sure is to ask her directly.