I don't put mine in a breeding trap. I just can't bare looking at the poor things being stressed. And I don't really want 50 fry a batch anyway. I catch what I can with the net afterwards, or when she is nearly done. This way you can also ensure that you are collecting mostly healthy fry only.
I wouldn't judge by the colour of the gravid spot. Mine have a dark gravid spot all the time. Some of them developed a big belly, some of them weren't big at all. This can only tell you how many fry she may give birth to.
Depending on the fish and colour, normally about 24 to a few hrs before giving birth the anal area opens wider, looking almost inflamed, giving the fish a square look. This is hard to see on a very black female guppy though, at least mine. However, most of mine start either racing up and down the tank walls, and then go to hide in plants or corner for a moment to drop the fry, then again swim frantically, then again hiding...Others become aggressive towards food a few days prior. Also, if there are other female fish in the tank(my males didn't seem to be big fry eaters), they kind of start behaving like sharks searching for the fry, scanning the bottom, swimming low., going to the corners where the mother was hiding, etc..If the fish has given birth, the males tend to chase her even more to ensure next generation is theirs

It's really easier if the fish has gold/blond body, as the gravid spot opening is very visible 12-24 hrs prior to birth(their gravid spot may look kind of reddish though, rather than black). It's like the anal area is going to birst, and the bottom part is kind of hanging down.
In general it really depends on the fish, her colouring, visibility in the tank,but when she gives birth once, count 21 days afterwards and next batch should be close. And the same fish tend to behave the same way the next time she is due.
Anyway, have fun with them and if you think you missed the moment, make sure you look really patiently at the gravel and plants as sometimes the fry are hard to see
