a woman that my boyfriend dated a few years ago
Say "HI, I am Ms Strong. It is nice to meet you" She is not an ex wife, just a girl he dated. Heck you may have met others & don't know it.
I have met women my FWSO dated, no biggie. He is with me, not them for a reason.
Ex's have moved on themselves. If there is an issue then the more awkward you are the more the situation will feed.
You are his SO now and enjoy! She is non-significant!
It'll be awkward for me if the "so how do you know X?" comes up, and she answers "We used to date"…I don't know what to say to that.
You say, "oh, cool" and then change the subject to something innocuous...like "how do you know our host?" or "beautiful weather we've been having lately" etc...Treat her like you would anybody else you meet for the first time at the party. Because that is all she is, someone you are meeting for the first time at the party.