By their behavior. When they're no longer puffing up like a turkey, lying on the ground like a fluffy pancake, making chik-chik-chik sounds, etc. you can try letting them out for a test. As long as they don't head back into the nest box they can stay out, though keep in mind they might be sneaky and go eat first, or dust bathe, then go to nest box, so you'll need to keep an eye on them or check on them for an hour or two. If they go back to the nest then lock them up and try again the next day.
