ok, well first you MUST request that the car be COLD when you go see it (go early in the AM).
-This is to check the coolant seals. They tend to expand when the car is warm and contract when cold IF THEY ARE DAMAGED. IF IT'S COLD then turn it on and step outside. Check the exhaust. If you see any WHITE smoke then the coolant seals are toasty (you have to take the motor apart to fix it and some just rebuild while it's all in pieces). If you see this then walk away.
get a compression tester You should get 90+ psi per face, per rotor.
It should have a smooth idle, no lumpiness, misses or vaccuum leaks.
Check the fluids, any and all rotary owners KNOW to ALWAYS keep the fluids topped off and frequent oil changes.
If it idles fine, no fluid leaks, and everything looks good, then go for a drive.
It should feel smooth and linear from idle to redline (don't forget to ask to redline first, nobody likes ppl abusing their cars, especially during a test drive) with no hesitations.
The rest is like any other car, suspension, tranny, etc., etc.
Look at this site for a more thorough list of things to check:
http://fc3spro.com/TECH/h2b.html
Good lluck and DO NOT RUSH INTO THINGS. my Rotary cost 5500 + 4600 to replace the motor a few months later.....I was NOT happy. Don't let it happen to you