a way to do it is move the shifter forks through each gear and rotate the input or output shaft and listen or feel for vibrations, catches, or clunks. You can hook it up to an electric drill to get it to spin a little faster. If there is oil in it, drain it and look for chunks. If you are really gung ho, you can pop off the top cover in short order, be mindful of the detent balls and springs (they like to run around), and look at the gears and synchros. The gears should not be pitted or missing chunks, and the synchros should have little brass rows of ridges on their insides and should not be smooth.