Okay -- what I meant to say is that the rule wants tanks under 1.32 gallons for altered only, but any size is okay. For modified it says tanks must be at least 1.32 gallons.
As for measuring how much they hold -- Bob, I have checked capacity while doing tech when, for instance, I've had the racer open the tank -- which was obviously questionable under the rule -- and then further saw that he had altered the innards of the tank to further skirt around the intention of the rule. I check by following the racer to the fuel station and watch as the tank is filled. For impound -- I don't work impound at SCTA/BNI events, so can't comment on how often it's done at one of those events, but I know that tanks at Maxton won't make it to imound if they're questionable -- because they'll have been checked during inspection.
If you feel the rule is stupid -- as usual, consider submitting a rule change form.
Racer X -- as far as the way the word "tank" is shown as plural -- I will assume that the writer of the rule did not imply that multiple tanks would be installed, but rather used the plural to imply "tanks on all bikes" rather than the multiple tanks on one bike. Here's a good instance where I'd buck up to a higher level of official for a ruling.
Is that a little more clear? I sure hope so. Busy day at the store -- that's my story. . .