"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.
