blinkers/headlights dying

I trickle charged my battery, took it to the shop and they said it takes a good load.....

So as I cruise on my bike, slowly,(after 20 minutes) yet surely, the battery is dying. My lights go dim, blinkers stop working, have to kick start it....

I'm guessing the alternator, or whatever it is called on a japanese bike, is shot. Is there a way I can decided whether or not that is working properly?.... and is there other problems it may be other than the alternator.