What interview question to ask when hiring an iOS Developer?

"So, what do you think about Swift?" followed by whatever questions it takes to elicit a real opinion. Any iOS developer should have a solid opinion on the language, either of the form "I'm sticking with Objective-C because X" or "I love it because of Y." Either is a great jumping-off point for an awesomely insightful technical conversation.

If they don't appear to have an opinion or clearly haven't thought/read much about Swift that's going to raise a serious flag.

Ask them what they think of Apple.

If they hate Apple with a furious passion, then they are an experienced Apple developer.