I think you just kind of know when they're ready...whether they just have more control over their body or they start waking up for no other reason than to be reswaddled (ahem, the last one was the clue for us).
With prop weaning, you kind of just have to be in the right place mentally, preparing yourself that, yes, it might be hard, and you might be at it for a few days before things iron out. We had 3 days straight of horrible naps...but it was totally worth it.
Also, just remember that she will need to get used to the sensation of having her arms out...eventually she won't be bothered by them moving about and will do all sorts of things with them (above the head, out to the side).
HTH a bit. Good luck!