same as a piston motor - put the compression plug tester into the spark plug socket, crank... you just have to use a very short probe, since rotary spark plugs don't stick out. pull one out, and you'll see. Normal compression ratio for a rotary is about 90-110psi, if memory serves
you can also check instructions here:
http://www.fd3s.net/compression_test.html
and tips here:
http://www.rx7club.com/showthread.php?t=386678