A couple of months ago, my girlfriend of 2 years broke up with me. I am still very much in love with her, but that's a whole different story I don't need advice on, however context is always going to be helpful with what I do need advice on.

As the title says, I have a FWB that I started seeing shortly after my break up. Both of us have no interest in dating each other, but we both enjoy BDSM play.

Then there is the second girl who I have had a romantic past with before, but nothing really happened when we first started dating, very briefly, 3 years ago.

A month ago I got in contact with her and we met up and hooked up. Since then I have met up with her again and hooked up once more. After the second time, she asked if I wanted to go on a picnic. Being curious about my first failed attempt at a relationship with her, and being recently single, I agreed. This suggestion, to me, makes me feel like she wants something more than random hooking up and wants to possibly start dating.

Assuming she is testing the waters to see if she wants to get in a relationship with me, I'm curious as to what point people feel it would be appropriate for me to stop seeing my FWB.

If we do start dating, I will definitely call it quits with my FWB, but I want to know what line people think needs to be crossed in order to determine when I should do that. Considering I don't know exactly what she wants right now, what should I look out for?

EDIT: The girl I'm going on a picnic with is a little shy and very awkward about upfront questioning. If she wasn't, I'd just ask her and then that would be that. Considering that the FWB is also relatively new, I don't want to ruin that by betting on possibly getting into a relationship with the other girl and then having nothing if it doesn't pan out.