Summarize this article in one sentence.
It may seem like a cliché, but finding a smear of lip gloss in a shade you don't wear on his shirt is a good sign that he's cheating. Smelling another woman's perfume on his hair or clothing is another warning sign. If you notice a lipstick stain or strange perfume on your boyfriend once, it may just be from a friend or relative, so you shouldn't necessarily worry. However, if it happens more than once, you may have cause for concern. When a guy starts seeing a new woman, he often starts to take more pride in his appearance, so he may starting working out more, using self-tanner, or whitening his teeth. If his grooming routine begins to change, it may mean that he's seeing someone else. A guy who's cheating may shower more often because he's trying to wash away another woman's perfume, so if your boyfriend is suddenly showering two or more times a day, you may have reason to be concerned. Most of us have our phones with us at all times, so there's nothing suspicious if that's the case with your boyfriend. However, if he never lets it out of his sight so he even takes it to the bathroom with him, that may be a sign that he's hiding something like another girl. Even people who are in a relationship deserve privacy, so it's never a good idea to go through your boyfriend's phone to look at the call log, text messages, or emails. That's true even if you suspect that he's cheating. There's nothing wrong with having a Facebook, Twitter, Instagram, and/or other social media account. However, if your boyfriend is constantly scrolling through his Twitter or Instagram feed, it may be a sign that he's no longer committed to your relationship.  If your boyfriend is a social media addict, he may be using the apps to communicate with other girls or even meet new people. Just as with his phone, you should respect your boyfriend's privacy even when you're suspicious. Don't try to figure out his social media passwords, so you can go through his accounts. That's a classic sign that he has something to hide, particularly if he always used to leave them open before. He may be calling or texting another girl and trying to prevent you from finding out. Even if your boyfriend isn't closing doors in order to communicate with other people, it can still be a sign of a problem in your relationship because it signifies emotional distance. If friends and family members that you trust start dropping hints that your boyfriend may be seeing someone on the side, you may want to evaluate your relationship and see if there may be truth to their suspicions. Always consider the source when it comes to accusations that your boyfriend is cheating. Some friends and family members may be genuinely concerned about you, but others may be letting their own baggage affect their perception. For example, if you have a friend who's recently had a significant other cheat on her, she may be more likely to accuse your guy, even if he's done nothing wrong.

Summary:
Look for obvious evidence of another woman. Take note of changes to his physical appearance. Notice if he is obsessed with his phone. Watch out if he's always on social media. Notice if he's constantly closing doors behind him. Listen to warnings from your friends.