How can I tell if my car battery is healthy and charged?

The starter battery on my Toyota Auris has failed twice in recent weeks. I have had to jump start the car using jump leads and another car. I suspect the starter battery has not had enough journeys recently to charge it up. (I do too many short journeys.)

The garage tested the battery and said it is not faulty. I have tried to charge it using a battery charger. (Can't really tell if this has charged it up or not.) Is there a way I can tell the general state of battery charge? I'd rather know if I'm likely to breakdown at the side of the road again, or not.