I've watched guppies go after their own fry. They are very fast to eat them. Endlers, on the other hand, do not.
Something to look for as a sign that your guppy is close to dropping her fry is when her body becomes squared off. If you look, even when she's pregnant, her body from the head to the gravid spot is still rather curvy. At the point when dropping the fry is very close, she will become so packed that her shape becomes more square than curvy.
I've also heard that some people can actually see the baby's eyes in the gravid spot just before the female drops them. I never could. But I have noticed that the gravid spot becomes much larger the closer the guppy is to dropping.
If you really want the fry, move her to a birthing tank and keep a close eye. The other poster is right; when she's starting to drop her fry, she will become very still close to the top (not necessarily at the top). She will drop a few and then swim to eat them, then drop more and try to eat them, repeated until she's done.
If you're able to catch her dropping the fry, you could put a screen up with large enough holes for the fry to fall through, but too small for the mother to get through. I would only do this if I was able to watch things so the female doesn't accidentally get trapped.
There's also a plastic birthing container you can buy. It's around 5"x3" or maybe a bit larger. It's designed to hand on the inside of your tank. The bottom has two plastic parts that create a funnel down to a slot at the bottom. This is for the fry to drop into so they can be safe below the mother.
I've never tried any of these methods, and yet, I ended up with fry. I found mine sucked up in my canister filter! Amazingly, it appears they can survive there for a few days.
