How to check an alternator in a Vauxhall omega?

Start the engine, put a volt meter across the battery terminal and have a helper bring the engine RPM up to about what it would be at 30 mph. The meter should show about 13.5 to 14 Volts DC. If it won't go that high, its an alternator or regulator problem. If it does, great, now move on to the next step. Turn on EVERYTHING, lights, blower motor, rear window defroster and watch the volt meter. It shouldn't drop significantly and if it DOES drop slightly, a few extra RPM should bring it right back up. If the alternator has a serious problem keeping the voltage up to that level, you should have the alternator tested by a qualified shop.