From the outside
A rev B board CoCo 2 can be most easily identified from the outside by the channel select switch above the TV output and a vertically mounted modulator (metal box) which can easily be seen through the top grille, just to the right of center.
From the inside
If you are able to open your CoCo 2, it is possible to confirm the board model by looking for REV B on the bottom left corner of the board, somewhat obscured by the keyboard. Also, you should see a 6847 to the left of a vertically mounted RF modulator can, a metal box that converts the 6847 output to be viewed on channels 3 or 4.