If you know this woman and she has no idea you are now her ex's partner ....why don't you engage her in conversation on a casual level ...you know the sort of thing 'where did you work before' (unless you already know) ....are you married, single, have an SO (sorry couldn't resist it ...I hate that expression significant other !) and if she has no idea who you are she may tell you all about her ex boyfriend ...should be fun ! Myself I have always been devious and for me it has always been fun and by far the best way when in any doubt !
I once snooped through my ex husbands brief case (no mobile phones in those days !) and found the name of a hotel in Brighton (of all classic affair places !) when he said he was in Norwich for a weekend conference. I phoned the hotel and the receptionist said ah, Mr AND Mrs ...have just gone out ...so I just said Please can you leave my husband a note when he gets back, to ring his wife ,,,,,the receptionist was a man and it was hilarious ......later that night he came back home and was full of apologies ..you know the thing 'oh I'll never do that again' (he did !) ...etc etc....never mind, I took my daughter out and we bought lots of new clothes and toys (she was only 5 at the time !).....he subsequently left and immediately the day he left I had all the locks changed . Divorced, sold the house (working for a divorce lawyer at that time who said YOu cannot have everything ! Watch me, I said ....and did ! a couple of years later I had a phone call in my new house some 200 miles away, from his girlfriend, now his wife asking me why I never told her he was a compulsive gambler as they had had their brand new house, a restored old vicarage in a lovely Suffolk villagem repossessed and were living in a Council hostel for the homeless ! BOY THAT FELT GOOD ! I had actually told that woman about his gambling many years earlier and was delighted to remind her .....revenge is sweet ! (Im not really a horrible person !)