Ive been worrying about the box in mine, but I think its just me not used to the heavy clutch and the fact that when I disengage the clutch without enough rpm (which is pretty much how I drove my 1.4) im liable to come close to stalling.
Ive found the best way is hover the revs at 3k and rapidly disengage the clutch to get a nice smooth & rapid takeoff.
On more serious note, while the car is rolling forward in gear, push the accelerator in hard. if the revs shoot up but there is a noticable delay in acceleration, then your box is most likely shot.