Put a multi meter across the battery with the engine off and check voltage, helps if it's a little flat.
Start the engine and hold the revs up a bit 2000 - 3000 rpm should do it. You should have a reading of 12.5 up to 14.5 volts. If not pound to a penny its the reg. you can get them from that internet site that Andy dislikes. But i never told you that
Check all the connections first though, don't forget the earths.