With the battery cover removed, you should see a connecter with three yellow wires on each side. One side goes to the alternator, The other side goes up under the faux tank to the rec/regulator. Many owners have already cut this connector out and soldered/hard-wired the connections.
If the plug is still there; with the engine off,disconnect it and mentally mark each wire on the lower half,(these should go to the alternator), A,B,C. With your meter set to the OHMs scale you want to read continuety on every combination of wires. A-B, B-C, A-C, and infinate resistance from any wire to ground.
If this checks good, then you can set the meter to AC volts and start the engine. You'r looking for between 40 > 60 volts AC on every pair of wires that you just checked. This test is wire to wire, not wire to ground... :gunhead:
Put a volt meter directly on the battery.
Record the voltage with the key off, it should be about 12.5V.
Record the voltage with the key on but bike not started, it should be over 12V if the battery is good.
Start the bike and record the voltage immediately after starting, it should be above your first reading.
After a minute or two record the voltage at 3,000 rpm, it should be 14V +/- 0.5V.
If the tests go well and you buy the bike then definitely check/bypass the connectors on the yellow wires from the stator.
I know everybody seems to want to do the Volt or Ohms method but I seriously urge you to try Monsieur Ampere's method, it's very easy and works perfectly with little or no electrical experience.
Get a hold of an Amprobe (clamp meter) any Electrician or A/C tech' you know will have one and you only need to borrow it for ten minutes, set it to auto range,generally that's all that's available, run the motor and clamp it around all three yellow wires in turn between the alternator and the rectifier, anywhere around that plug behind the side panel will do, if it's still there, but those wires anyhow, rev the motor to 3000 rpm, do it to all three wires individually and if all the readings are thereabouts same, you've got yourself a good alternator.
Providing of course, all three readings are not zero.