Well the first tip off is that you are wondering. In most relationships it doesn't come up. So if something is making you wonder its probably your intuition pin balling around in your brain. I am writing this answer as if we are talking about a man who is cheating but the same rules apply to women. After fourteen years of being a Hairapist I have noticed a disturbing trend in women cheating. Its like Gabby and Bree from Desperate Housewives made it fashionable or something. Difference being that men often have affairs for years that they don't even tell their best friends about, while women brag to the check out girl at Rite-aid as if they were telling you about finding this seasons Prada heels at Marshall's for $20. Its bizarre.
But I digress. How do you know if your crush is married?
Clues:

- You only have a cell phone number and a personal email address for him. He only calls during the day, rarely on weekends and travels a LOT. In the event you get to travel with him you don't get to sit together because "the company bought the tickets so he has to pay for yours separately". Why is there always a mix up with that? I'll tell you why. His assistant bought his ticket and she didn't know he had a travel companion. He bought your ticket on Expedia and is crossing his fingers that the old lady sitting next to you will be willing to switch seats. If you order room service he isn't hungry. He eats half your dinner but the bill only reflects one meal. Go figure.
- He only takes you to places that are out of your area for dinner. He prefers small romantic hole in the wall restaurants that are at least 30 minutes from home, dimly lit and not "Trendy". Thereby lowering the odds that he will run into his wife's best friend's sister who might mention that he saw you out together.
- Even though he was just kissing your neck, while holding your hand and telling you how beautiful you are the minute he spotted a friend in the distance he began treating you like you were the first carrier of the next Pandemic.
- In the unlikely event he doesn't say " Hey I'll be right back" and intercept them before the friend can get to you he will politely introduce you as " Susan" or whatever your name is even though he threw out the word Girlfriend to you earlier in the week. He is now making sure you are out of arms reach just in case you make an overly affectionate move toward him.
- You have never been to his house. Probably because his "roommate is a slob and its embarrassing", it is "under construction and simply not safe" or he is "staying at a friends place until he figures out his next move". But mostly because his wife lives there and she is really weird about him having a girlfriend. God she is high maintenance.
- Though he thinks you smell terrific; he is allergic to perfume and thinks you are much prettier without lipstick. This is actually because his wife's instinctual pinball machine has been dinging ever since he came home smelling like Chanel No. 5 and is now checking his collar for lipstick daily. So unless you happen to like her fragrance and lip color your gonna need to dial it back.
- You have had sex in the car, a hotel, your house and possibly even after hours at his office but he has never spent the night. Probably because he has sleep apnea and has to wear this really goofy looking apparatus when he sleeps and its embarrassing. His wife is used to it.
- He is never available on holidays. Valentines weekend he always has a business trip with his boss so he cant take you with him. He will totally make it up to you that he has to be out of town on New Years eve the weekend before or after. At least he always calls to chat for a few minutes that day to tell you he is thinking of you. That hushed voice is kind of sexy.
- You have never met his family. His mom is kind of "crazy" and his kids aren't really ready to see their dad in a new relationship yet. He really likes you and wants to make sure its the right timing when he tells his family about you. Like six months after his youngest child graduates from college and he finally asks his wife for a divorce. Too bad his kid is only 3.
- He lowers his voice and leaves the room when he takes a call. Then comes back and tells you that there is some urgent matter that he has to deal with at work immediately and he is so sorry but he has to go. But he will totally make it up to you later. On a week night.
- And finally He is very vague about how he spends his time without you. Its probably not that interesting to you and he would much rather talk about more important topics like which position you like best and if he can meet you at your place during lunch for a quickie. . .
So there you have it. If any of these are setting off warning bells ask him or her straight up if they are married. Then insist that you go to their house and meet their coworkers. If they wont do it - get out of there! You are an awesome person who does not need to share. If you find yourself believing that this might be the real thing and maybe he just married the wrong person, remember the only thing you can be sure of is that this person is a cheat who lacks the integrity to extricate themselves from their current relationship before embarking on the next one. You will probably find yourself finding out what it feels like to be cheated on by this guys sooner than later. Best case scenario you are the rebound relationship and that one has about a 2% chance of working.
You deserve better than that.