Search their trash mail or look for patterns. If you think that "something's not right" with your current relationship and you have the impression that he or she is not completely honest with you. Most of the time someone who is hiding their dating site use will be a pro at deleting their computer history. Therefore, this strategy works better for married people, who want to rule out or confirm any dating site activity.
Save time, avoid dishonest relationships, and find your perfect partner.