Yes, everybody muddles through these things differently
If dating somebody in an established couple, they probably already have some agreements between them. I'd start by asking what agreements they have. In my case, my married boyfriend just had agreement with his wife to let her know when he starts dating somebody, and when he is becoming sexual with somebody, I know he gets texts and phone calls a lot when he is with me - I rarely text or call, and have settled things mostly by telling him it's up to him to tell me if I do anything communication wise that is a problem for him or his wife, and I am glad to change my behavior if that turns out to be the case. I try to just communicate as naturally as I would as if it were a monogamous relationship.
My husband and I have many more agreements/steps when starting to date somebody, but that is OUR onus to communicate to other romantic interests, not their job to worry about if they are stepping over lines or not.
If you end up meeting their partner, it can help if you are able to talk to them, and let them know you're open to input from them about how they feel, and that if they have any problems they are welcome to talk to you directly. That might be overthinking (cause thats what I do!), but if the worry goes on long term it might help, if that feels comfortable to do.