Well, you'd slide the cassette on and there would be freehub sticking out the end. When you have both cassettes off, just set 'em next to each other on a table. You should be able to tell right away if they're the same size.

As to not having to adjust the RD, you would really need two wheels with the same hub. Otherwise, odds are the spacing will be ever so slightly different. But you could get lucky. Only one way to find out. I went through this with my two wheelsets for my CX bike-- I had a regular 10/11 speed hub on one set and an xD driver on the other. There was a huge difference between the two-- it was a 5 minute adjustment every time I changed wheels. So I finally broke down and bought an xD driver and cassette for the other wheelset. It's sort of the nuclear option, but I don't have to mess with the RD anymore.