No, it explicitly stated in the text that Maithanet can see the few and even Proyas knows this. Ain't got time but if you need me to I will quote.
Some confusion is going on here so lets slow down a bit.
Can the Few see the Few?
Kind of ... let me explain. The Few can see the Mark - this is why there are tests, like the Wathi doll, that schoolmen use to recruit new practitioners (normally children). You aren't marked until you actually use magic.
Remember that Achamian was shocked when Kellhus told him he could see the Onta, and further shocked when he was able to awaken the Wathi doll. If "the Few can see the Few" such a thing would have been obvious. So, no the Few can't see that other's are able to see the Onta, what they can see is if they have
used magic and have the mark.
Was Maithanet of the Few?
It actually might not have been explicitly stated but its heavily implied. Its of little consequence as he doesn't use magic.
How did Moenghus know Kellhus would be of the Few?
The dunyain use educated guessing - they are not omnipotent. That's why its called the Probability Trance, and the thing with probabilities is that sometimes things happen that are unlikely.
Moenghus knew that he himself was of the few (obviously) and likely surmised that there was either a genetic component of it (there is), a training/physical competent, that it was a mark of intelligence, or some weird combination of all these things. Given any of that, or all, or some of each, either way he guessed that any son of his but most any dunyain as well would be one of the Few. To answer your question, did he know? Of course not. He didn't walk up to Ishual, have a chat, run some tests on his son, etc. He made an educated guess.
The whole thing you guys are getting into with dreams is only tangentially relevant. Yes, both sender and receiver must be of the Few. I could be wrong, but I don't think Moenghus sent dreams to Kellhus. Part of the trouble with that method is you must know the location of the reciever - my guess is that the Pragma and other Leaders tended to sleep in the same place while the other peons did not. Moe sent dreams to everyone that he could find, shotgun approach to make sure the message was delivered.
Did Moenghus' plans demand Kellhus was a schoolman?
Hard to say. My guess is that they would have worked up through Shimeh regardless, so it doesn't matter. Kellhus didn't really need, or use, magic until he killed Moenghus - which I assume was not part of Moenghus' plan (that's a whole different debate though). Keep in mind Moenghus died because he got the probabilities wrong, just like Maithanet, just like Kellhus. The Anasurimbor, the Dunyain, have a long history of guessing wrong and their plans ending in catastrophic failure - they make mistakes.
Did TTT demand that Kellhus was of the Few?
Depends. Whose TTT? Moenghus'? Probably not, Moenghus was going to unite with his son and unite the world. Kellhus'? Yeah, probably, but I would imagine that if he wasn't of the Few he would have either devised a different plan or simply been subsumed by Moenghus. Non-Magi Kellhus is much less impressive.
Hope that clears things up.
Re: NC - There's no reason
anything has to have meaning. Yes, its a fantasy book, but every word and event is not suffused with importance. This book/series specifically has red herrings all over the place. Its perfectly valid to assume something is as it appears. Not saying its right, just that "because its a book" isn't any better reasoning to pick or discard a possibility.
That said, I'm also aware my opinion on this particular subject makes for an uninteresting conversation, so I'll just watch as other's hash it out. If I think of anything exciting to contribute either way, I will of course interject

.
I think sometimes we get caught up in debating right vs. wrong (my theory vs. your theory), instead of agreeing to a shared set of assumptions for a topic and seeing where it takes us
(I'm probably more guilty than anyone of that). It might be more revealing if we could say "hey, for this topic, lets assume X is true. How does that affect the world if it is?", instead of needing to debate X every single time.
Great topic btw, turned into a fun thread.