Here are a few signs that I have come to recognize that may be warning signs of something going on that shouldn't be...but by all means, this doesn't always mean that they are cheating.
1. difference in appearance, the way they dress, different cologne, spending more time in front of the mirror, better grooming habits.
2. not coming home when expected, spending more time out alone or with friends, becoming more interested in being somewhere other than home or with you/family.
3. becoming more secretive, hiding cell phone or cell phone bills, credit card bills, receipts, deleting incoming or outgoing phone calls, not being able to be reached, not answering normal questions about their whereabouts like before.
4. noticing your partner getting more calls or email than usual
5. becoming less interested in being with you or doing family things together
6. less sex, less intimacy, or wanting to explore more sexual positions than before
7. discovering lies that they have told you.
8. picking fights so they can get out of the house with a reason when normally they don't leave after a fight.
9. your partner not wearing their wedding ring when they normally do.
10. noticing that your partner is acting differently towards you.