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 also be 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 any way to discern the true nature of the child?
Best Answer
Any boy is not a hag
Note that hags are always female, the description you already quote says (emphasis mine):
So any child that is a boy can be safely assumed to not be a hag.2
Most things that only affect humanoids or fey and fiends can work to distinguish girls
In 5e, each creature has a type:
Human children are considered to be of the humanoid type (emphasis mine):
Despite their humanoid figure (head, torso, legs and arms), in 5e Hags are either fey or fiends, as can be seen this D&D Beyond search (Shago is not actually a hag, I promise) and the hag children should therefore be fiends or fey as they are said to merely appear to be human (emphasis mine):
Using abilities that only affect humanoids or that only affect a fey or fiend can therefore be used to distinguish humanoid children from fey or fiend children. Since some of these abilities require saves or do not always work they aren't sure ways to tell. Here are some examples, note that this is not a comprehensive list, just some illustrative examples.
1.Abilities that you can use to be absolutely sure:
2.Things that can help identify hags but you can never be absolutely certain due to the possibility of saving throws or other limitations:
Like I said, this is not intended to be a complete list, any other ability or spell that affects humanoids but not fiends or fey or vice-versa can help in distinguishing hag children from humanoid children.
1. Suggested by sirjonsnow
2. As noted by DrTrunks Bell in 4e and 3.5e this was not as conclusive since male children of hags were possible through a different method and known as Hagspawn