[Deaf Smith]

"tell by smacking the bottom of the mason jar"

Those true their craft carry a hydrometer to show the proof of their product.

Measures the specific gravity of the alcohol.

The greater the SG, the higher the proof until max weight obtained.

[Mariner]

“The greater the SG, the higher the proof until max weight obtained. “

Alcohol has a lower specific gravity than water, so the reverse is true.