Eyeball

To estimate a quantity of something, usually drugs, based on sight and without a scale or measuring instrument. Sometimes another object, such as a coin, is used as reference to compare the size of the stash with what the true amount is or should be.

Jack: 'Dude, how much kush do we have left?'
Will: 'From Eyeballing it, I'm guessing we have a little over an eighth left.'