FWIW, the way i found out (and it was completely inadvertent) was checking our cell phone records (our son has Tourette's syndrome and some other issues and I imposed some phone limits on him so i was checking his calls/texts--- in the process noticed my wife was making about a zillion texts/calls a day to a friend of mine (whose number i had committed to memory and recognize on the online billing record). Cell phone records are, IMO, fair game, especially if you are the one paying the bill. See a frequent number you don't recognize and these days it's pretty easy to check it on reverse phone look up or white pages or whatever.
Prior to seeing the phone records, my W had been showing a lot of the same signs your W and others WWs on here have shown... going out late with GFs more often, dressing more provocatively, working out/losing weight, phone-guarding, but i was pretty clueless. Honestly, I was kind of done with MR, too, at that point. It wasn't until I saw the phone records and then overheard a call between her and OM a couple of days later (Again, completely unplanned/inadvertent) that i was shocked into waking up.
In my case, the burner phones, both times, i just got "lucky" and found when she was careless with them. Straight Talk wireless (carried by WalMart) is the primary offender, here... After the fact in both cases i actually found the receipts from her purchases, which she again carelessly left laying around.
Other than getting lucky when they slip up or by checking the phone (and maybe Credit Card--but she'd have to be really stupid to, say, be getting hotels or anything on your joint cards) records, not sure what you can do. If she has an android phone and a google account and hasn't thought to shut down her location tracker and clear her history then google tracks everything... and i mean EVERYTHING. Downloaded and uninstalled apps (What'sApp is a popular one, and a big red flag, though there are many others), web searches, location history, map searches... in some cases if you haven't turned it off it even saves speech to text and in some cases even voice messages and conversations. All of that stuff is accessible from any computer with an internet browser if you have the google/gmail password. Not sure if apple phones do the same or not. Bottom line, here, though, is that if she has been careful there are ways around all of that stuff.
Only way to be really sure is to hire a P.I., but that can obviously get pricey. Alot of the stuff they use (small voice recorders, GPS trackers) are available for purchase even on Amazon, but are of uncertain legality depending on your jurisdiction. Probably best in those cases to leave it to a PI if you can, but, if you can't, be EXTREMELY careful and know what the law is.
Other than that, I'll repeat again: 1) Dont go searching for something you think you wouldn't be able to handle if you found out and 2) Once you DO find out, don't keep looking except in narrow, spot check-type circumstances where appropriate... it can become an obsession and, in the end, also make it harder for YOU to return to the marriage if the info you find is too scarring.
H53/W51, R-ing 4/'18
"Do not arouse or awaken love until it so desires"-Sg.of Sg 2:7
"So oftentimes it happens,that we live our lives in chains, & we never even know we have the key"-Eagles III 1:3