The approach I take really has little to do with the agile buzzwords, but it does have to do with agile practices. One of the commonalities in all agile teams is the short iteration, most people get that part (it's one of the 12 principles behind agile on the http://agilemanifesto.org site). The purpose of the short iteration is to get feedback early on the quality of the software developed. This is where I start.

1. Ask about unit testing. Overwhelmingly the answer I get here has been "uh, we cut that out because we didn't have enough time" (note: first 2 warning flags--no time and no unit testing)
2. Ask about when the software was tested, and how often. Answers can get creative here. Particularly if the team uses "Agile" as an excuse to throw all process aside. If the answer is toward the end of the project, or anything other than with each iteration, they don't know what agile is.

So far, I haven't had to go any further than this to know that the person doesn't know what agile is. I've also only been in one interview with a company that already had well established agile processes in place.

There's more than one way to do agile, and I care more about the principles of agile than any particular brand or buzzword.