One person in season 1 that I definitely think knew was Jack...especially after getting held hostage by Chakotay (I'm sorry I can't see Robert Beltran as anyone other than his Star Trek character!)...his 'crazy' comment sold me on the idea of him figuring it all out, but to be honest Clark kinda made a fudge of hiding the secret from him!
I think of him as Chakotay, too, and I barely watched Voyager. It's just so much easier than trying to remember the character's name.
As far as Jack goes, if nothing else tipped him off, it was Clark breaking out of his handcuffs. I really don't see why Clark has such a problem just saying, "I'm not Superman," or whatever the situation calls for. He tells so many other lies constantly that there's no reasonable reason for him to object to lying about that outright. All in all, I'm a bit puzzled why Clark is such a bad liar. He has been acting all his life, pretending to be a normal human and lying about his capabilities, so he should be good at lying about his powers by now.
