Any boy is not a hag
Note that hags are always female, the description you already quote says (emphasis mine):
A week later, the hag gives birth to a daughter who looks human until her thirteenth birthday
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:
A monster’s type speaks to its fundamental nature. Certain spells, magic items, class features, and other effects in the game interact in special ways with creatures of a particular type.
Human children are considered to be of the humanoid type (emphasis mine):
Humanoids are the main peoples of the D&D world, both civilized and savage, including humans and a tremendous variety of other species.
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):
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.
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:
- Detect Evil and Good can be used to detect feys or fiends:
For the duration, you know if there is an aberration, celestial, elemental, fey, fiend, or undead within 30 feet of you, as well as where the creature is located.
- Forbiddance damages (and might have the side effect of killing) certain kinds of creatures you choose:
Choose one or more of the following: celestials, elementals, fey, fiends, and undead. When a chosen creature enters the spell's area for the first time on a turn or starts its turn there, the creature takes 5d10 radiant or necrotic damage (your choice when you cast this spell).
- Trying to create a Simulacrum of each of the children is a (very expensive and lengthy) way to ensure they are humanoid since it only works on humanoids or beasts:
You shape an illusory duplicate of one beast or humanoid that is within range for the entire casting time of the spell
2.Things that can help identify hags but you can never be absolutely certain due to the possibility of saving throws or other limitations:
- Paladin's Divine Sense also allows you to detect fiends (but not fey):
Until the end of your next turn, you know the location of any celestial, fiend, or undead within 60 feet of you that is not behind total cover
- Arcane Abjuration from the Arcana Cleric's channel divinity only affects feys or fiends:
As an action, you present your holy symbol, and one celestial, elemental, fey, or fiend of your choice that is within 30 feet of you must make a Wisdom saving throw, provided that the creature can see or hear you. If the creature fails its saving throw, it is turned for 1 minute or until it takes any damage.
- Hold Person and Dominate Person are examples of spells that only affect humanoids and therefore can be used to rule out children that are affected.
- Zone of Truth1, Detect Thoughts and other methods of compelling creatures to be helpful or learn their thoughts are also of some limited use, depending on what the children actually believe/know.
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