Is it possible to tell if a child will turn into a Hag?

There’s been a string of kidnappings and my players are about to discover a Hag coven. In the Hag’s lair they will discover several children prisoners as well as some hag children. All will be the missing kids.

I’d like the group to be able to figure out which ones are which before returning the kids to their parents. What to do with the innocent looking hag children (and who won’t become hags for another 4-7 years) should be an interesting dilemma for the characters. Of course if it is impossible to tell, that’ll be also an interesting dilemma.

The MM p 176 says:

“Hags propagate by snatching and devouring human infants. After stealing a baby from its cradle or its mother’s womb, the hag consumes the poor child. A week later, the hag gives birth to a daughter who looks human until her thirteenth birthday, whereupon the child transforms into the spitting image of her hag mother.

“Hags sometimes raise the daughters they spawn, creating covens. A hag might also return the child to its grieving parents, only to watch from the shadows as the child grows up to become a horror.”

This doesn’t seem to give any guidance on telling if a child is a hag or not. Is there anyway to discern the true nature of the child?