I don't think there are any hard and fast rules, but I think a simple question that will give you an immediate sense on whether a designer knows their sh*t (pardon my French) is to ask them what their design process is. If there's any hesitation, or if there's a pause and you can tell they're trying to make something up as they go along, they don't have one. Move on. But just pay attention to what they're saying and you'll likely be able to tell how experienced they are and how confident they are of their skills.

Edited to add: this is where you'll find out if UX is part of their process. If they begin with things like mood boards, font selections, color palettes, look & feel, no, no and no. It needs to begin with some form of audience/customer, business goals, content strategy, user flows, functionality. Those other things come after, not first.