How to tell condition of Gearbox

Hi all, gearbox i've got fitted in me 90 at present has the problem of not going from 5th to 4th - already come to the conclusion that it'll be the syncro on that side. Now i've managed to get hold of another box off good old ebay - externally looks good, input and output shafts look in excellant condition (no wear) and no obvious slack when turning them by hand - i've done what us lads do and wobble the gear lever, no strange noises, goes into all the gears ok. and the good old 'click' is very audible when coming out of 5th towards 4th.

Guess what I'm after is if anyone knows anything I can check before chucking it in landy, or are we talking best to strip it . . i'm tempted just to chuck it in a see )

Cheers fellas!!!

drain the oil out of it and see if theres any bits of metal that come out with it for a start .