He didn't. He automatically assumed it was the Potter's kid. I don't think that he really thought of the Longbottoms much. And plus I'm guessing the time that would make Neville's parents memorable would be when they were tortured insane. But that wasn't done by him but rather Bellatrix, her husband and his brother.
He thought of the Potters as a threat, seeing that they were both very skilled and close to Dumbledore, and in the Order. So you must assume that their child would be a skilled wizard too.
Harry is also considered a half-blood, since Lily is muggle born and mudbloods arent really wizards/witches, in the eyes of those who care about blood purity. Voldemort and Dumbledore are both half-bloods, and Voldemort feared a third half-blood prodigy like himself and Dumbledore.
Kendra Dumbledore, Albus' mother, is a muggle born witch like Lily.