SORRY ABOUT THAT, I MISSED READ YOUR POSTING AND THOUGHT YOU MEANT A 100 POUND TANK. A 100 GALLON TANK WOULD OR SHOULD BE CHECKED BY THE PROPANE COMPANY. I AM SURE THEY WOULD DO THAT FOR YOU BEFORE FILLING IT UP.
Not that I ever tried what you are attempting, but I do have several 100 Pound tanks and have them filled at the local hardware store. They will take a tank and fill it with approximately 23 gallons of liquid propane. I am pretty sure that you could take a tank that you put regular air into and it would be purged out when they fill the tank.
The old soapsuds trick might work (I have used it to check connections in the past), but I am not sure if I would trust it or not? I would think you could construct and fixture so you can place a pressure gauge you screw into the outlet fitting. Once attached open the valve and check what the pressure is in the tank. If it holds pressure for a period of time, then I would say the tank isn't leaking.
Suggest you call someone that fills tanks and ask them what to do. They might even inspect the tank for you.
Another thing you could do is to take it to a place where they swap out tanks. TSC does it, but the cost of their tanks is higher than what it cost to have a tank filled, but doing it this way you will end up with a newer tank and the worries go away. They will check the bottom for rust and the tank for dents and what have you, but the 100 pound tanks are not required to have the new type of fittings so your old tank could be swapped out.
