Pretending like you're a CIA agent spying on terrorists makes this more fun, promise.
Now, the overall key here is to be patient. Observe for a while. It's going to be really tempting to just get sucked in and go through everything as fast as possible, but a quick daily undetectable check of his phone will give you a lot more information than blowing up at him over one text that he'll try to lie his way out of. Collect evidence, build your case, and then bring his world crashing down.
A lot of this is iphone-specific, but hopefully still helpful!
1. Go through his phone. If he's the type to always have his phone on him, try getting to it when he's showering, or distracted in a way that you'll have a warning (e.g., water turning off) to exit out so you don't get caught. If the phone is password protected, watch him enter his password and use it when he isn’t around. If you have trouble doing that, wait till you're cuddling on the couch and he has his phone, and be dorky and send him a "hello!" text, and then watch.
If he has an iphone, double tap the home button so you can see what apps he most recently had open, and make sure you leave it in the same order. It's important to leave the phone exactly as you found it until you’re ready to confront him. Getting caught before you’ve gotten your proof will just make this harder.
Now go through texts, photos, browser history, or whatever you like. You can text screenshots to yourself, just delete those texts when you’re done! (Tap edit in the message and just check the texts you want to delete individually, don’t delete the whole conversation! Also see step 4 for tips on getting a more thorough texting record)
While you’re on his phone, do yourself a favor and go into his email and search for “password” or “pw” – having his computer and itunes passwords will make the next parts easier. Just make note of any passwords you find, people often use the same passwords for everything.
2. Go through his email. You can do this on the phone too, but it’s much easier to search on the computer. Most people have their email set to automatically log them in, so you just need some uninterrupted time with his computer. If you need his password, hopefully he’d emailed it to himself in step 1! If he has gmail, first log him out of chat so people don’t see that his account is active and try talking to him, as this could lead to awkward questions. Use the search bar for names of girls you’re suspicious of.
If you want to save copies, either forward them to yourself and delete them from his sent items, or print them as PDFs and save them to a flash drive.
While you're on the computer, might as well check his facebook and internet history, as this could give you more clues! Just don't forget to wipe your own browsing history. This is super easy in Chrome, you can just check each log you want to delete.
3. Use his phone to stalk him. Think he’s lying about his whereabouts? Make sure “find my iphone” is turned on on his phone and log into his icloud account (using passwords acquired in step 1).
4. Get a copy of everything. This is the most techy thing I did. When you plug your iphone in to your computer, itunes backs up everything. Everything. Now, you can’t just open SMS files on his computer, unfortunately, but a handy little program called iphone backup extractor can! It’s like $25, which to me was worth it, because I found out that his cheating went back way farther than I’d have imagined! I also have a bunch of nude photos of the girls he was cheating with, which I don’t plan on using but I guess could come in handy?
So, to do this I recommend using a flash drive (probably just 2GB or so), plugging it into his comp and transferring the files to your own computer. If he has a PC it should be under a filepath like this:
C:\Documents and Settings\(username)\application data\apple computer\mobilesync\backup
The file will just look like a bunch of random unreadable junk – that’s where the software comes in. Download it from iphonebackupextractor.com, then use that to decode the file. You’ll get an excel doc with all the texts (timestamped with phone numbers), and one with the contact file, so you can cross reference names from the contact file with the text messages. I used conditional formatting to highlight different girls in different colors, because I love color coding even when I'm color coding skanks!
You can also go through photo attachments…but brace yourself for this, I definitely did not expect to see what I saw and it was a bit shocking. Who are these girls who send nude photos to guys they barely know? Oh right, skanks.
I used this last trick once I’d gotten enough evidence elsewhere, but just needed to satisfy my own curiosity about how much he was really lying to me (a LOT), how long he’d been up to it (a LONG TIME) and how awful of a person he really is.
Best of luck, and let me know if you have any other tips to share!