Depends which part of the transmission, Dom.
Gearbox - Jumps out of gear when driving, usually when increasing load. Grinding or rumbling noises when a bearing is failing. Inability or difficuly in selecting a gear when the selector forks are worn. General noise emanating from the gearbox area.
Clutch - Usually slipping when the pressure plates/friction plates are losing their grip. Slip level increases with load. Rattle from a worn, bearing type, pressure release bearing.
Propshaft/driveshaft, usually vibration when the universal joints are starting to fail, rattling, machinegun like rattle, mainly on turns, when the Constant Velocity Joints (CVJs) are on their way out.
Differential, usually a loud whine, varying with load, when the worm/bevel gear/associated bearings are worn.
Just a few for starters, Dom
Ken.