No, I forgot you can lock it. Next time ;D
You could also put a quick disconnect on the gas side and run some tubing into a small bucket of sanitizer.
With a closed off keg, how do you guys know when it’s full? I’ve done only one push with CO2 to a keg, but I ended up taking the lid off so I could see when it was just above the 5 gallon line.
Calibrated volume markings on the buckets.
I do this in two ways. Since I have a stainless steel fermenter I can’t see the level in the keg or fermenter. I try to cold crash the beer prior to kegging down to just above 32*. Then because of the temperature difference between the beer and the outside air, condensation forms on the outside of the keg at the level the beer is at. When the condensation hits the top the keg is full.
The other way, in cases where the temperature difference isn’t sufficient, I use a bathroom scale. A full keg weighs 50.5 pounds.
Cool - I’m going to go for the scale method because I usually tip my better bottles up sideways prior to racking so marks wouldn’t help. Thanks!
Or beer shooting out of the PRV.
They would if you tip the fermenter 90° to the markings.
Well I tried the method with the scale today but it would help to have a scale that doesn’t automatically shut off after a certain amount of time. 
It worked out fine just following the condensation and coldness as it krept up the side of the keg.
Condensation is an easy way to tell how full your keg is as long as you crash cool before transferring (and don’t live in the desert?).
If you don’t want to fill above the gas in, you can connect a gas QD with some tubing on it and use that as the vent instead of the pressure relief valve. Also works if you have some kegs with non-removable PRVs.
Beat me to it! 