Maybe start by getting to know them over dinner and discussing other things. Like, find out if they do hunting or fishing or canning or camping. Those kind of things can be a sign of a prepper mindset. If it’s reasonable, talk about the latest power outage in your area. But keep in mind, if they are preppers, they’ll probably be just as wary as you.
You could also start by sharing or offering to help with something… sharing abundance from your own garden or chickens, or asking if they’d like help getting the harvest in, or whatever. Forming bonds will help and you can get a sense of whether they’re a “taker” or a “sharer”.