Wanna know how _I_ would measure the volume of fluid remaining? (ask a physicist)
option 1:
I would find the total mass and then the mass of the container itself. From the difference, we would know the mass of the contents. Next, determine the temperature of the contents. Thirdly, determine the specific gravity of the contents. Lastly, using the relation between density and volume, determine the volume using the specific gravity (density) and mass.
Not the least bit practical in this case.
option two:
1) Find the total volume of the container geometrically
2) fill the container until it is full, noting how much fluid it requires (using the same compound as the contents of the container).
3) subtract the volume required to fill the container from the geometric volume
4) account for a 0.05% difference due to material displacement (contianer)
5) have a beer and be satisfied that I know how much volume _was_ in the container
option 3:
1) insert a hose into the container
2) drain the contents into a number of vessels, adding the volumes as required
You see, simply put ... I am far too lazy to determine the volume of the contents using calculus. I am sure that the shape is very unruly and would be a pain in the ass to integrate - this said, I doubt very highly that the vessel is actually a perfect cylinder and is in fact a streched cylinder (more like a rectangle with rounded corners). Meh!