HOw to test fuel capacity?

Try to measure the tank(s), and here you can calculate the volume:
http://www.sensorsone.com/length-width-and-height-to-volume-calculator/

Not to many options. First try checking with the builder, they should have records and be able to tell you what they put in the boat. Unless they went belly up ...

Measuring the tank dimensions is an option if you can. Tanks are often partially buried..

If the tank as sight tubes or a top access port so you can use a dipstick, you can bring it down to the minimum safe level you d be confortable with and then fill it up. It will not give you the actual capacity but the safe / usable capacity

Last option would be to hook up a transfer pump and empty the tank into another one before filling

Or maybe somebody here owns or runs a sister ship and can confirm tankage

The last thing I d use is a broker or YW listing

Just a reminder that there is an air gap at the top and often the tanks cannot be sucked completely dry which amounts to about a 20 percent difference between measured volume and usable capacity. My tanks are stated to be 300 gallons but I begin to suck air at with 222 used.

20% is a lot, far more than I ve seen on any boat i ve owed or run. Sounds like the pick up doesn't extend far enough.

Agree, 20% is a lot to haul around.
(Long shot maybe, but did you look for a reserve valve or switch that will let you suck up the rest? On one of my boats I had a similar un-usable capacity but it was to protect the system from sucking up dirt and dead algaes from the bottom of the tank. If all the fuel was needed for an emergency or whatever, the pick-up tube could be pushed all the way down to the bottom of the tank)

Never had to use the feature, but I would also take the tank apart and clean the insides every 5 years. Windex and paper towels worked great, although not very "Nautical". I am sure West Marine will sell something really expensive to do the same job..

Well I am a bit lazy so I d rather use a much fuel as possible then empty the rest in one or two drum to check amount then top off the tank.

Much less work than lining up 40 drums to measure a 2300 gal tank

Having just finished pumping 40 gals of oil in and out of the boat I can't imagine doing 2000 USg and yes the dock neighbors were watching ...

Yeah, we have a gear pump to transfer fuel from the stern tank to the main and its 15 minutes to move 400 USG.

I still don't want to line up all those drums