Maybe it's just me, but reading your OP makes me think that you could be talking about two different scenarios of a de-catted DP. Either it's completely catless or has been gutted.
Catless DP, the DP was either built without a Cat or the Cat was cut out and replaced with same length of pipe. It should be fairly easy to look at the pipe from the engine bay and underneath to visually see if there is a bulged area (this is where the cat would be). If there is no bulge in the DP to the mid pipe chances are you have a catless DP.
Gutted DP, the catalytic materials were removed and the shell of the catalytic converter remains in tact on the DP. If your DP has a cat and you need to find out if it's been gutted... the only way to know for sure is to pull the DP off.
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'12 Prius Plug-in