How to determine between Smart keys?

Hey guys,

I lost one of the fobs for my 2006 Prius. In researching buying a used fob, I discovered the following:
Black logo fobs are not smart keys.
Chrome Logos are smart keys.
2004 smart keys will not work with my 2006.
My 2006 fob has all the same numbers as a 2004 smart key.

That last tid bit is the one that worries me. I already purchased a fob on ebay because they claimed right in the title that it is 2005-2009, so I have an automatic case with paypal if it doesn't work. However I am just wondering if there is any way to tell the difference between the newer and older fobs? Thanks.