Practice. With experience and practice, one can get pretty good at estimating quantities. The Scouts use to teach (do they still?) how to estimate measurements and also taught to use body parts to help, e.g., the width of a hand, the length of a knuckle, etc. Liquid volume measurements might be a little trickier but knowing the capacity of gear you carry or, e.g., how much is in a half-filled baggie, will let you get away from carrying a (single purpose) measuring cup.

That said, I carry a clear, Lexan-like insulated mug that has the volume levels clearly marked. Mine is from Campmor but I've seen the same thing at REI (just different logos). I really like the mug...it's fantastic at keeping things hot for a very long time. The one down side -- it weighs nearly 6oz. (Thermo Measuring Mug #82540-B $4.99)

If you or others choose to go with this mug, here's a tip: The plastic lid from a canister of Progresso bread crumbs fits the mug perfectly...providing a tight, secure seal. It adds a little bit more to the insulation and keeps the creepy crawlers and flyers out of your hot chocolate. The color even matches the markings on the mug.

FB