1. He takes his phone with him every time he goes to the bathroom and often puts it on airplane mode when you're together or looking at something on his phone.
2. You barely spend time with his friends. The chances are they're probably aware of his wrong-doings, so spending time as a couple with his friends becomes less and less frequent.
3. He puts off spending time with your family and friends. Guilty conscience, anyone?
4. He loses track of details or gets things wrong. His memory of two people's birthdays/mother's names/shoe size blend together and he starts to forget simple details about you.
5. He's very secretive about money and budgeting. While you're constantly asking when you can spend your savings on booking your next trip away together, he's blowing it off after spending a lot of money with someone else...
6. He picks fights with you and accuses you of things that you both know aren't true, because he's trying to project doubts onto you.
The Love Coach TJ Gibbs explains: "His accusations that you're jealous and/or crazy [are a sign]. Attack is a cheater's best form of defence. These are classic ploys for making you doubt yourself. Unless you are a habitually jealous person."
7. He has more work responsibilities and suddenly has to stay late a lot. It's an obvious but a very tell-tale sign.
8. He absolutely FREAKS OUT when you pick up his phone, even if you just want to check the time or google something mundane.
9. He becomes less affectionate. He's happy to switch off the light and go straight to sleep without a second thought.
10. And also doesn't want to have sex as much, because he's getting his fill elsewhere.
Beverley Stone, author of Stay Or Leave? Six Steps To Resolving Your Relationship Indecision explains: "It's a major sign that he's having sex elsewhere. Tell him you need to see a sex therapist. That will scare most men! Have an honest discussion about what's going on, and how you can make time for a romantic evening and sex."
11. He gives you guilt-gifts. Rather than supporting you emotionally, he thinks he can buy you loads of shit and make up for his guilty feeling.
12. His mood swings become more severe because his other relationship's problems are beginning to have an impact on your life, too.
13. He begins to care much more about his appearance. You find his food baby endearing—but if he's trying to impress someone else, he might start obsessing over the gym or get a new haircut and not even care if you notice.
14. He lies about unnecessary things. Once he's got into the habit of lying to you regularly, telling little white lies become easier and easier. "No, that's not another woman's perfume" easily becomes "Yes, I called my mother" and "I promise I only had one bottle at the bar."
15. Your gut is telling you that he's cheating on you. There will forever be a text post floating around on Instagram about how "girls always find out everything," and so if your gut is telling you that something isn't right in your relationship—trust it.
This article originally appeared on Cosmopolitan.co.uk. Minor edits have been made by the Cosmo.ph editors.