Take the seat off and you'll see the sender unit mounted in the top of the gas tank. Check the 2 connections, The unit is secured with a metal lock ring that's fairly easy to remove with a hammer and blunt edged punch or screw driver.
Remove the seat and disconnect the fuel level sensor that's right on top of the tank. If your gauges stays at full, chase the wire out from the sensor to the meter to make sure it's not shorted to ground. If the meter does go to empty when you remove the wire pull the sensor out and check it with an ohm meter as already mentioned. You should see a resistance (on the low resistance scale) change smoothly from 0 ohms with the float arm all the way up to around 10 or so ohms when the arm is down. I don't remember the exact resistance but it was in that neighborhood. If the sensor stays at a 0 ohm reading it's got a problem. You may be able to fix it, if not you'll have to replace the sensor.