The only way they might know they are refilled is if you write the date and weight on the outside of the tank. I mark mine and weigh them so I know how much is in them. They all weigh about 450 grams, about a pound, empty. When full they weigh about 970-1000 grams or about 2 pounds.
I was being polite about the DOT rules, as I don't want to cause any trouble for the site host. I believe they are there to prevent people from making a business of refilling the tanks and transporting them in commerce. I may be off base, but aren't these rules just for commercial vehicles?
The biggest problem I've run into is keeping the valves clean. If you get crud, rust, or sawdust into the valve area it could be carried into the gas appliance and plug it up.
The tiny orifices are very prone to blockage. I always blow a little propane or air through a valve before connecting it, to blow out dust or bugs. Keep the little protective caps for storage, and give the tanks a good sniff test a few hours after refilling and before putting them away for storage. If they smell they are leaking. A little soapy water or spit will show you where. Don't leave soap or spit in the valves.
Anyway, I use propane motor fuel in one of my rigs and it's convenient to fill my BBQ tanks using the liquid line from the big tank, and fill the smaller ones from the BBQ tanks.
