If you don't have a full 4 racquets you want to demo, then just do it. If you do have a full 4 racquets you want to demo, either do 2 in the 3/8 and 2 in the 1/2 or, even better, find someone you know that has whichever you don't have and hit with their racquet for a few minutes. It's not perfect, but it's best to find out what grip size you want before you demo.
I'm in the same boat as redmini. I have 5/8, so oftentimes I have to buy a 1/2 and build it up. If you are in doubt, I'd go with the smaller grip when it comes to purchasing a racquet. While it adds weight, it's a lot easier to build up a grip than it is to get it smaller.