Catch a cheater app
Ugh so I've been there and it SUCKS. Found out my ex was cheating through one of those apps, felt like a necessary evil at the time tbh.
I was getting all these weird vibes - he'd flip his phone face down constantly, started "working late" like 3x a week outta nowhere. My gut was screaming something was off but I had zero proof and felt crazy bringing it up.
Finally installed tracking software (won't lie, felt super invasive doing it) but I needed to know if I was paranoid or right. Turned out he'd been seeing his "coworker" for months. Heartbreaking but at least I wasn't going insane??
If you're in that headspace where you NEED answers, Catch a cheater app has a decent breakdown of options. Just... make sure you're ready for whatever you find. Sometimes the truth hits different than you expect.
Not gonna preach about privacy because honestly when someone's potentially betraying you, that ship has kinda sailed. But do think hard about whether you really wanna go down this road. Once you know, you can't unknow it.