I got a call from a friend who saw my last post here, and asked me why I hadn't just looked in the catalog entries to see if a device was WFA certified.
It's because some of them lie.
If you want to know if a particular device is certified, get the device model number, AND version (A, B, v1, v2, etc) and look it up on the WFA website. http://www.wi-fi.org/search_products.php?lang=en&advanced=1
It is possible that older products that once had a cert are no longer listed, but generally speaking if it isn't there, it isn't certified.
Now with that said, it doesn't mean that a device that isn't listed isn't any good. It might work fine with duplicates of itself. Or it might work, but not be fully compatible. Or it could be great, but the company making it is too small, and can't yet afford the WFA membership, testing, and certification fees for it.
I've actually had more trouble with Bluetooth devices not being what they say they are. My best example was a Class 1 (that's hi power) dongle with an external flip-up antenna. I got it cheap, took it home, and it only worked as well as the class 2 model I already had. Decided that I might as well add an RP-SMA antenna to it, and opened it up to find that the "antenna" was just a piece of inert plastic made to look like an antenna. The space inside the case, behind the original antenna swivel point, was just empty space. I guess I'm lucky it worked at all.