I think a walker could permissably subconsciously pick up something and with enough time and familiarity grow to suspect someone might be a walker.
I kind of figured the familiarity would be a key factor, based on the one example I could think of (Raleris talking to the kids at the end of his story about Vasilias, though that was more hinted to the audience than stated outright). Not that I think that's a bad requirement, but the direction I was writing before I thought to question it was to just "see" it in a stranger's eyes or something.
The oldwalkers could, absolutely, recognize latent sparks though their accuracy with it wasn't great. In canon Freyalise recognizes Jaya's spark but mistakenly thinks it's Jodah (nearby) who's sparky. In the M:EM I had Ellia recognize Halea's latent spark in "Wind and Void" figuring that even if such sense was lost in general, Ellia in particular is ancient, has studied the nature of the spark, and was looking for it.
Which has me wondering, how long has it been since the Mending, again? hasn't it only been like 20 years or so in canon? Regardless, I'm kind of of the opinion that, by their nature, oldwalkers have a different set of character drivers than neowalkers, and I don't particularly want to go down that route.
As far as studying the spark... I'm not sure whether that would be a point for the character, really. It
could fit in to what I have so far, and the overall greed or lust for power that the character has, but I haven't quite nailed down whether they're... intellectual? enough to rigorously study it.
I don't have a canon answer for that, but personally, I don't see any problem with it. I guess my instinct is to make that the special "thing" about a particular 'walker. It sort of seems like each 'walker has some "thing" that sets them apart from other 'walkers, whether it be Illarion's ability to bring someone with him through the Eternities, Denner's ability to Delve, or Alessa's unique chronomancy. I certainly don't see why one could have the ability to sense the spark in others, even a latent one.
I think if you wanted to spread it over multiple planeswalkers, though, that wouldn't be as good of an idea. It would become like the Highlander sense thing, which, I mean, I liked for that, but not so much for the Spark. For one thing, we've had numerous stories where two 'walkers sort of need to feel each other out, trying to figure out if they were or were not 'walkers, and if suddenly they can just sense each other, that would kind of nullify all of those stories.
But as a character's special trait, I could definitely get behind that.
That's pretty much why I was asking, since if this isn't a "thing" that most 'walkers can do, then introducing it as something common among them would be a problem. At the same time, the reason I was asking was that I was writing for a character who's
already got a big "thing" that makes them unique, though it's more their race than a specific power. I am simply
loathe to layer on abilities for the helluvit, because even I find that so much "extra" makes things boring.
As a for instance, there's an SCP that's this kind-of skinless beast which can perfectly mimic human speech in order to lure prey (seemingly not intelligent enough to understand what it's saying), which by itself is interesting, but then there's so many
other aspects layers on top of it that it becomes uninteresting: it's seemingly undead because its internal organs don't function, it doesn't actually
need to eat and just throws up when its lungs get full, it gives birth to human children which eventually morph into it, hints that
pieces of it can also speak or telepathically communicate - it's all just too much that doesn't feel compelling, at least to me.