Most guys hopefully won’t have a similar experience to the one Luke Wilson’s character has in Old School, when he arrives home early from a trip only to find his girlfriend cheating on him. The truth, though is that the odds of your girlfriend or wife cheating on you have increased over the years, according to an Indiana University study. In the 2011 study, 19% of women admitted to cheating compared to research showing that 10–15% of women reported being unfaithful in the 1990s. (If you're wondering, 23 percent of dudes admitted to cheating, according to the study.)

The most common reason a woman would cheat is because she feels taken for granted, according to Charles J. Orlando, a relationship expert and bestselling author, who went undercover as a cheating husband a few years ago to find out the reasons why women cheat, and Lisa Brateman, L.C.S.W., psychotherapist, and relationship specialist. So your best bet to prevent ever getting cheated on in the first place is to keep your effort high throughout the relationship instead of just when dating, notes Orlando.

Still, if you and your partner haven’t quite been in sync as of late and think something may be wrong, here are some signs that she may be cheating on you. The key word here is "may." If you can relate to a few of these, consider confronting her about it in a calm way—and definitely don't immediately accuse her. “To stay in a good place: Listen without judgment,” Orlando says.

1. She’s not sharing details about her life with you

Anyone who's ever told a lie understands the risk of getting caught when people start pressing you for more details. With each added layer of falsehood, you’re that much closer to exposing yourself as a liar.

So how does that translate to relationships? "What you're not talking about can be very telling," Brateman says. "For example, if she comes home from a two-day conference and doesn’t talk about the conference, that could be a clue that something's up. People do not like to talk about something that hasn’t happened for fear of being discovered."

2. You’re no longer her go-to guy

It’s also a big deal when you’re not the first or even second person that she talks to about her day, according to Brateman. “It likely means she's talking about all that to someone else,” Brateman says. “And sharing personal details of her life with this other person who truly listens and encourages her only deepens their emotional connection.” Brateman says when a spouse used to tell you about a co-worker or friend and then stops talking about them entirely, chances are something may be up.

3. She looks noticeably different

“Cheaters have the tendency to change physically,” Orlando says. “Love handles disappear, posture changes, makeup gets applied daily, and their muscle tone/definition returns.”

So, if your girlfriend who used to wear sweats and no makeup to do errands is now getting dressed up and doing her makeup before leaving the house to go to the convenience store, that might trip a few alarm bells, according to Orlando. The key factor here, though, is whether these changes are sudden and drastic, Orlando says. “Sometimes, it's not necessarily what they're doing, but what they are doing differently.” Brateman says.

4. Your sex life isn't the same

Most people think a sign of cheating would be that you’re not having as much sex as you used to. But a more accurate warning sign, Brateman and Orlando explain, is that a couple is dramatically changing how they're making love.

“A sudden interest in experimenting with new positions or techniques in bed, or interest in things that she wouldn’t consider before,” Brateman says, can be a sign that she’s having sex with other partners and bringing it back into your bedroom.

5. She’s extremely defensive for no reason

”When you calmly tell your partner that something feels different, and she responds in an aggressive way, it may be because she is deflecting or hiding something,” Brateman says. “In a loving relationship, your partner wants to know how you're feeling. This is part of the process of resolving what feels different or uncomfortable.”

Brateman stresses that if there isn’t trust in the relationship, it won’t be able to survive or grow so the onus is also on you. “The first thing to do is to stop seeking constant reassurance after you have been reassured,” she says. “This puts pressure on the relationship without any benefit.”

Bottom line: If she gets angry in an instant or acts overly nice, it’s a red flag, Orlando says.