Peresonally, I'd define what you need them to do, and test whether or not they can do that. Whether or not they have their CCNP isn't as important as that, I think. Sadly Vince isn't far off about some people getting that credential, somehow, without knowing very much.
Having said that, we just recently saw a big change in the CCNP program, which makes this test a bit tricky. It's possible to get it now without going through the QoS, remote access or security stuff. :/ If I were making such a test I'd just give them a few diagrams and config samples, asking them to explain what was happening, whether or not it would work and whether or not it could be improved upon. Include as broad a range of the curruculum as you can, and ideally tailor it to projects similar to ones you'd need them to work on. If they seem weak on anything or don't answer as fully as you'd expect, follow-up with more specific questions about the technologies they didn't give you much information on. You can be nice about it while still being an effective interrigator.
For the actual credential, just ask them to go on their Cisco.com account and "publish" their credentials via their email credential-sending thing. It's secure, and better than the certification code itself since that doesn't confirm if it's current or retired.
CCNP, CCDP, CCIP
