How do you easily test an interviewee's "Aptitude"? Especially in a developer sense. I struggled with how to do this when asked to help with interviews in the past.

Yeah - sadly, it's extremely subjective. In the beginning, I hired some lousy people and some great people, and eventually I got much better at quickly and accurately sizing up someone's competence, independent of their personality. The shy people need some softball questions to help them feel relaxed; the bigger personalities need to be poked and prodded a bit to see how they react under a little bit of pressure. A good team has a mix of different personalities, and if they're all bright and eager, they'll get along just fine.

Anyway, to answer your actual question: One thing I'd always ask at some point is, "Can you tell me about a time where you solved a hard problem?"

They'll think you're trying to gauge their problem solving abilities, but their answer will give you much more valuable insights regarding their aptitude and enthusiasm. While they're answering, you'll get to ponder:

"What does this person think is hard?"

"How well can they communicate a complex topic?"

"Does solving hard problems excite them?" etc