Tom,
Here is how I did it:
Before my tanks had ever had any fuel in them, I leveled the airplane and put a measured 5 gallons in each tank. I then disconnected the fuel line at the carb and used the electric pump to empty each tank. I noted the time 'til the last fuel came out. I then measured how much fuel came out. I did some math and found out how many gallons-per-hour the Facet pump was capable of putting out, and how much "unusable" fuel was in the tank. On my RV-6, the unusable fuel was .5 gal per side. Since this fuel should always be on board I weighed the aircraft that way.
If wesyou have fuel injection I think you could disconnect the line to the fuel servo.
Oh, and don't forget to ground the aircraft and fuel containers.