As Duncan says simply turn on the heater on - select to demist the windscreen as this will definitely turn the aircon on - you should hear/see the fan startup.
There is a cheap fix/bodge where you swap the wires round so as to use the brush for the medium or fast instead of slow - effectively turning your fan to 2 speed. As, especially with the diesel, these will very rarely be used.
I had mine on a T4
and discovered all three speeds on mine had gone, so I bought and fitted the Kenlowe kit from Jules.
One word of warning if you do leave your fan broken for a long time expect your aircon condensor to fail as well as it will overheat. I bought mine with this condition and ended up having to replace some aircon pipes, seals and the condensor. Plus a pressure test and regas of the aircon all in probably cost me an extra Ã‚Â£150. So worth checking and sorting asap.