Beware of defensiveness. Your partner may get very defensive whenever you ask them if there’s something suspicious going on. For example, if you ask your lover how their day was, and the response is “Why are you in my business?”, you might be dealing with a cheater. What’s most important here is an unexpected unjustified defensive response to the most basic of requests. If you get into a fight if you ask someone what they were up to, where they went, who they were with or when they are getting back, you may be dealing with a cheater.