Depending on the age of the tank there are two over fill indicators that go off at 80%
Modern tanks have a float type valve. at 80% it closes and filling ceases. HARD.
Older and modern tanks have a "Visual Overfill indicator Valve" Yes that is the name of the valve.
Many people, will tell you you need to open this valve to let the air out of the tank becaus you can not compress air... I have no idea where thy went to school but...Well.. I will forgoe some serious insults
(1 Air is easily compressed.. I mean what is filling your tires and 2: No air IN the tank aftr the first (PROPER) filling,,,only propane liquid and gas and some contaminats).
When that valve starts spitting liquid (Comes out as a white cloud,like 40 degrees below white cloud)the tank is 80% full.