There are a few clues that can help you determine if someone is only interested in a hookup. One sign is if the person avoids talking about anything serious or personal. They may also be more aggressive in their pursuit of sex, or they may make it clear that they only want to have sex without any attachments. If you're not sure whether someone is interested in a relationship or just a quick fling, it's best to just be honest with them and see what happens.