Tests can be done, like the Wathi Doll test that Akka used to test Kellhus. That's a great example because it shows that Akka couldn't identify Kellhus as being of the Few the entire time they were together, and it shows that sorcery can be done without marking an individual (albeit extremely minor).
I'm sure the Mandate had several such tests, and Kellhus could probably devise his own in the event that a child of 3 years old couldn't be coerced into saying the name of a Wathi Doll
.
Kellmomas was never tested because Kellhus just didn't care at that point. You might also argue that he didn't test on purpose so that he wouldn't have to bother Esmenet - such a loving husband.
I suppose that is the most likely possibility, then.

She might have indicated to someone that she can see the Mark, while Kelmomas never did.
Also a good point! It would be just like Kelmomas to keep that secret piece of knowledge to himself... And, like Wilshire said, it was not like Kellhus cared much at that point.
That's a great point. Akka, when his Eyes Opened, he saw the world in aching clarity, full of magnificent beauty. When he sees people marked, he sees a wounded world, repulsion at the scars upon reality.
A child, unable to speak but able to See, might cry hysterically when held by such a scarred monster.
For some reason, I am extremely amused at the mental image of Kellhus noticing how little Serwa would have a terrified reaction anytime she saw him or someone else with the Mark and thinking "okay, there might be a pattern here"...
As we've surmised elsewhere, Achamian vaguely refers to a test done by the Mandate Pederisk which identifies children Few. It very well might be the Wathi Doll test but it could be as simple as putting three objects in front of a child and asking them to pick out the "different" (Marked) one.
I had completely forgotten about the test done by the Pederisk. You're right, that was most likely what they did, or something quite similar. Especially as that was already established as being specifically used for children (unlike the Wathi Doll?).
Though the children the Pederisk tested were usually older than Serwa, right? I seem to recall Achamian mentioning he was like eight or nine by the time he was identified as being of the few (might be misremembering, though).