Keep a journal of your partner’s reported activities. Write down the times, dates, places, other people involved, excuses given, etc. Your journal will become invaluable as you compare what’s said with phone bills, credit card statements, ATM withdrawals, talk to other people, etc. A cheating partner is likely to change his or her story, or question your memory, so keeping a record of everything is critical.
Keep track of all incoming phone calls. Record the time and number of all calls.
Go through the trash, wallet, purse, pockets etc. when they are not around. If your partner is going to restaurants, bars, movies or anywhere else with their lover there will be receipts and they will want to get rid of them. Many times they will get sloppy or hurried and throw them in the trash at home or hide them in their wallet or purse. If you find anything that you are suspicious of or cannot explain save it as evidence or write down the details and put the receipt back where you found it so that he/she does not get suspicious.
Plan a surprise visit to work, or come home at unexpected times, or make announcements about having to work late, but then come home early, etc.
Keep track of your partner’s mileage, receipts, credit card statements, atm withdrawals (unaccounted for cash), phone records, etc.
If you can, check your Partner’s call log. Look for an unusual amount of phone calls. Keep in mind that cheating spouses often store their lover’s phone number under someone else’s name: a friend, a co-worker, etc.
Never confront your partner until you're certain that you have enough evidence to make your case. And never reveal all of your evidence at once. Most cheating partners will try to concoct a story to fit the evidence presented. But, if you withhold some evidence, and let your spouse create a story, it gives you the opportunity to use the remaining evidence as leverage. And by strategically withholding evidence, your spouse will start to question exactly how much you know, increasing the odds that he or she will tell the truth.
If you find anything suspicious, do not confront your spouse until you're certain that you have enough evidence to get a confession.
Think for a minute about how your partner might try to dismiss your accusations (e.g., we were just joking around, I was just flirting, it was a misunderstanding, we are just friends, nothing happened, etc.). If you can anticipate how your partner is likely to respond, you can try to gather the evidence you need to counter what he or she says.
The most important thing to remember here is to be diligent in uncovering evidence , take detailed notes and save everything.