One method of inspecting a manual transmission when it’s removed from the car, with out disassembling, is to remove the side cover (shifter cover). Now you can inspect most of the gears and synchronizing rings.

The only problem is you need some experience in determining what is good and what is bad.

You can shift the Trans thru the gears by hand, place the transmission in neutral, all the shift levers should be in a vertical position. Neutral is when you can rotate the input shaft while not turning / holding the output shaft stationary with pliers. Turn the input shaft by hand at the same time shifting the Trans by moving a lever forward, that’s one gear, move the same lever in the opposite direction, which is another gear, place that lever in neutral/ vertical position. Do the same for the other shift levers.

What you are looking for is any grinding or clicking noise. Any roughness when in gear and if the input shaft is it very hard to turn; this is when experience comes in. Also check the Trans oil for any metal, if its looks like a slivery color that’s metal.