Really about all you can test is whether the booster gets vacuum. We have a special adapter to measure the vacuum after the check valve (NOT REMOVABLE) at the booster. During proper application there will be around 20 in of vacuum which will drop about 5 in when the pedal is depressed and very quickly return to original vacuum. There should be no further loss of vacuum as the brake pedal is held. It should expell vacuum each pedal stroke, but the level shouldn't drop below 10-15 in on one stroke. Every booster can be pump "dry" so to speak by rapid pedal application.
Without being able to test after the check valve, I would remove the line from the booster and place your thumb over it and run the motor. Check the vacuum while teed into the source (leave all the rest of the systems alone) nipple on the line. Remove your thumb and the vacuum should drop. Replace it and it should reach full vacuum in seconds.
These boosters are very trouble free so used can be a reasonable option, but their wholesale price is so low that you might want to check around before going to the labor of used. The greatest percentage of bad boosters are those that have sat open for great periods of time - such as in junk yards.