Alternator probs on my TD5

Hi, when driving my TD5 auto the battery light comes on (only sometimes) when idle for example like waiting at traffic lights. Driving, no problems at all, never not started and today I took the battery to be tested and it seems that it's fine. Is there an easy way to test the alternator or do I have to remove it and take it to a specialst, any other checks I should do to eliminate possible causes?

Thanks
Al