I just try the wheels on the bike.

Or, if say going from 27 to 700c, and wanting to build from scratch, then find a good complete wheel, maybe from a different bike. Try it out, so you'll know what a similar wheel will be like once you build it up.

I am wanting to try some Compass 2.3" tires on one bike... if only I can figure out which bike. So, I went to the co-op and bought a couple of cheaper tires that were close in size. I got halfway into that when I discovered that something was terribly amiss. Thus, trying something else. Still, I want to get the co-op tires to fit before dropping the cash on new Compass tires.