Your spouse should mention their friends from time to time. It’s possible that they will have friends who are female and male.  If you find that they only mention friends of the same sex, they may be avoiding mentioning friends of the opposite sex. It’s understandable that they won’t talk about them frequently but if you don’t know a single person of the opposite sex that knows your spouse, there might be something strange going on. Furthermore, they may not want you to know any person of the opposite sex that they are affiliated with. This is not a definitive sign, but it’s definitely a warning signal.