See, I don't think Andalites can just "tell" when someone's infested with a Yeerk. Ax had a hunch because Jake sneered at him involuntarily, and he tested it further by pressing his hand to Jake's forehead and making Temrash scream like a banshee at Ax.
It was just dumb luck, really, because any of the kids could've been a smart Yeerk acting scared...or maybe Elfangor just sensed they were sincere in their obliviousness, but not in their Yeerklessness.
I always thought it was pretty stupid how so many Yeerks in the series just yelled "Andalite!" >.< Come on, of course it's an Andalite. Do ya need to give yourself away right on the spot?