The best way is to put a wheel in there. You don't need an inflated tire on there, but if you don't want to deflate the tire for wheel removal and install every time for the life of the bike, use a wheel with an inflated tire to test.

I've ridden setups that required deflation but didn't mind because I was usually pulling out a flat anyways. The worst is catching yourself trying to put a freshly patched, fully inflated tire back on the bike. Uggh!