I'm guessing that the fuel is DDD ABC mix?

Lets say it is.

If you could measure the mass of 100ml of the suspect fuel at say 20 Degrees Celsius, then we may be able to get someone else to measure the Mass of 100 ml of fresh DDD fuel at 20 Degrees Celsius. Measuring at 25 Degrees C would be just as useful provided that both fuel Temperatures are the same.

If the Masses are different then we can confidently say that some of the Diethyl Ether has evaporated. We can then easily work out a rough approximation of what Volume of Ether may need to be added from it's Specific Gravity.

If a plastic container of diesel fuel is loosing Ether then I'd be inclined to think that both the remaining Kero, Castor, and Ignition Improver would be degraded as well. Would you leave a container of Kero exposed to the air? Same with Castor. I wouldn't, I'd expect both to go off.