You could also use a battery hydrometer to check the specific gravity of each cell. This method will reveal if you have a dud cell.
If the battery is clear or even slightly opaque , then its easy to spot a sulphated cell - they are the white ones. If you have one of these, the battery will need some 'Inox Battery Conditioner', you can get this stuff at most battery places, its cadmium sulphate *I think*, and it reverses the chemical reaction that causes sulphation. Just bung 5-10ml into the sulphated cell, and top up with distilled water as needed, then give the battery a good old charge.
I have recovered / revived a few old batteries with this shit, it is great.
There is also a product out there that apparently deals with sulphated cells as well, it 'blasts' the battery with voltage spikes

sounds painful, I am not sure how effective it is , RedDave may be able to advise on this method.
Easiest way to tell if your charging system is 'charging' in the first place is start the bike, turn on the headlight, and rev 'er up - if the headlight noticeable brightens, your charging system is working OK. This method is only a rough guide, using a multimeter across the terminals is more accurate.
After these tests, it becomes a matter of using the workshop manual to test all the relevant resistance measurements in the alternator, rectifier etc etc. to see where the problem is.
If its a cheap Taiwanese battery ( Elephant, Yacht, etc) - its stuffed, get a Japanese-made Yuasa.
