Is it though? If you called up an old fling from 2 years ago, you're telling me there's a 50/50 chance of hooking up again vs just to chat? I very much doubt that.
No, I meant who is DTF asap vs who is DTF after some feels-good (small talk, catching up, etc) is exchanged
IMO if a DTF calls/texts out of the blue, high probability they are looking to re FWB or at least DTF
In OPs situation, it's possible sending a "hey Monica! I've been thinking of you, how are you? Come over so we can get together and catch up!" (Feels good)
Then if she comes over, see if she's DTF
It's generally all in the approach