Ask them how they handle design. If they tell you there's no design in agile, they aren't getting it.

Ask them how they manage changing requirements. If it sounds like changing requirements has its own process, they probably aren't getting it.

If they're claiming to use Scrum, look to see how they write it. Shops that do Scrum well tend to know well enough how to write it. Hint: it's not SCRUM.

That may seem like pedantry, but I'm firmly of the belief that in order to successfully apply a process template like Scrum, RUP, XP, or whatever, you need to understand the philosophy and the "why" so you know how to adapt the "what" to your organization. In Scrum, most folks who are doing their homework will come across that little bit of info. Folks who are looking for cookbook recipes for project management will usually miss that detail.