That's why you need to head on over to Infidelity DNA Testing and hire them to carefully examine your soul mate's dirty underwear for incriminating DNA.
"We're not exploiting misery, we're just taxing paranoia!"
Infidelity DNA Testing is an easy three-step process. First, you simply steal some of your partner's unwashed underwear and send it in for a test. While you are sitting on a park bench somewhere, staring off into nothing and wondering how your life got to this point, they will check the underwear for foreign DNA and offer you the final step: comparison. If there is a third set of DNA in those Underoos, you've got yourself a cheating spouse! Then you can talk it out like rational adults. Really, they'll have no choice but to be rational after you begin the conversation with "Speaking of taking out the garbage, guess whose DNA the lab found on your panties?"
"Oh I dunno, Karen, maybe it was your mother's -- wait, what?"
Oh, we should probably mention that all three steps together costs over $600. But don't worry, Infidelity DNA Testing assures you that this is way less than the cost of a private investigator. Though they conveniently fail to mention that it costs about $600 more than a simple sit-down chat with your significant other, who is undoubtedly a terrible liar.
Unless your significant other is kind of pricey, in which case you have bigger things to worry about.