TL;DR what are the right tactics to fish for info on what a girlfriend does on her Girls' Nights out and-or dissuade her from certain friends / doing certain things.

At a party last Saturday, my long term (<1 year) girlfriend is chatting with two of her best friends, and they go into woo-girl mode 'omg let's do a girls' night out in the city next Saturday'. I could have sworn I heard the girlfriend suggest they invite some guy and crash at his place after (or just crash at his place; she referred to it as 'using' him with a golddiggery laugh. This is unlike her. So I thought, at least.) Their own places would be about 45 minutes away. One of the friends dissuaded her. I don't know what their current plans, if any, are. It wasn't clear our serious they were vs it being a burst of woo-girl-ism in the moment. I was visible and within earshot of her; I don't know whether she was cognizant I was listening.

She knows I might be out of town this weekend, so given that she hasn't invited me it could be because 1. she wants a night out without me 2. there are no actual plans or 3. she expects I'm out of town.

This is extremely unlike the girlfriend. Compared to other girls and girlfriends she's very conservative about not fraternizing with other guys. I wondered if I misheard her (I was kinda drunk.) My questions are:

1. How should I react internally (i.e. should be I be confident she has no intention of cheating, given that she's "good" otherwise, or is this a Slut Red Flag)
2. What, if anything, should I say to her, and what kind of tone/approach should I use? Should I try to dissuade or ask beforehand? Or just ask afterwards? PUA relationship advice usually seems to say 'never seem jealous or threatened or weak' but also that sometimes girls want to be put in their place and told 'no'.

Now, I know that you get jealous. For a long time I wondered "How do they do it? The guys that NEVER gets jealous..." I then realized EVERYONE gets jealous. It's whether you have the confidence not to act upon it that counts. Tell her to enjoy her night out with her friends, in fact, encourage her to go. Don't be clingy and needy, just hang out with some of your friends and let her have a ball.