But do i confront her based only on this gut feeling that i have? I have nothing else.
IMO you can't "confront" her based on your gut alone. Heck, even when you have proof.. even rock solid proof... a WW will often just deny it ("Who are you going to believe, me or your lying eyes?") or try to argue around it or in some way put it back on you ("You SPIED on me!!"). I've been through it... I know.. and i've seen others here go through it as well. That's why, if you think you need it and can handle it, you get your proof, but then you never, NEVER share with her exactly what you know (because she'll just argue points about it or flat out deny it) or how you got it (because then a) she'll try to turn it back on you for snooping/spying and b) it burns that source of intel if you need it in the future.) Best thing is to just get the intel/data you need... and then stop looking, because you already know what you need to know. Then, when you confront her, you need only say "I know. I wont have an open marriage. I want you out". If you need to drop the odd detail (such as OM's name, or whatever, it probably doesn't matter) but the more detail you offer up the more you invite discussion/denial, which is completely unproductive, and the more you make it about HER and not about your boundaries... which is what you need to establish and enforce. You can probably establish such boundaries even without knowing for sure if she is in an A: "W, I know you have been spending alot of social time on your own... i don't know what you are doing, but I won't live in an open marriage or "share" you", but You obviously have a bit more weight on your side if you have some proof of infidelity, as well as, obviously, having been alerted to the sitch so you can be "on guard" (read the threads on boundaries for help, here... I was pretty bad at it initially.)
In my case, I flat out caught my W with a burner phone. Twice. Once when she had not turned off the buzzer, left it visible, and OM called her on it when i was standing right there and i picked it up. She had denials both times, even though what she was doing was clear.
I also faced denials when she had been up to no good and i knew she had been. I eventually learned that there was no point in continually policing/confronting her, and established my boundaries that i wouldn't share her or be in an open marriage. Later on, when she got caught continuing to contact OM (and, actually, after nearly an hour of sitting silent together in a car in the park, admitting/confessing to it) I indicated i was going to walk, and she offered to restart counselling, destroy the burner phone, and "no contact" OM (which she had already kind sort committed to before-- like i said at first i was not great at formulating/stating/enforcing boundaries. Even thereafter, it turned out she had been entertaining regular calls from OM at work, even as they were not "seeing" each other... a situation i would not have found out about had i not kept the options open on some of my surveillance by keeping details from her.
At any rate, Readers Digest version: If you think you can handle it, my thinking is that it is best to know, but, once you know, establish your boundaries (and enforce them, when appropriate) and stop looking. Keep the option open to spot check her if she indicates the A is over and/or otherwise wants to stay in the house or if she otherwise indicates she wants to come back to marriage and work on the MR but, otherwise, you know she cheated on you and the MR is, for the time being, over. Once you know that, there is no further reason to check on her unless you suspect she is violating a boundary or if she wants to "come back" and you need to spot check her.
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