My house has steam heat and one of the radiators is way too small to heat the room it's in. I acquired a good-sized used radiator from a neighbor, who had it sitting in her garage since the time she bought the house and didn't know if it worked or not. The price ($0) was right, but I want to test it before I go to the trouble of cleaning it up and installing it.

Google took me to this thread on this site which describes a good way to test if the radiator is sealed:

Sure you can test the radiator before installing it. See the inlet and outlet threaded connections? Plug or cap the one outlet with teflon tape on the threads. On the inlet install a bushing (use teflon tape on the threads) to reduce the size to adapt a pipe nipple, a tee, an air pressure gauge to one branch of the tee and an air hose fill valve to the other branch of the tee. Fill the radiator with seven pounds of air and wait an hour to see if the gauge pressure level drops.
Sounds good, but I will need to shop for parts and searching for "air hose fill valve" doesn't get me anywhere. I can understand from the description what it does but is there another more common name for it?