Make sure the seat's high enough that you can angle it slightly downwards at the front so your hips are higher than your knees.
Also the seat needs to be deep enough so that the whole of your leg is supported and your knees aren't about 6" beyond the edge.
You need to be able to do both of the above with you feet flat on the floor.
Chances of getting all of these without actually trying the thing out are pretty slim. If you're buying blind then get one with as much adjustment as possible to give the best chance of being able to set it up right.
This video's pretty good: