To hydrostatically test a pressure vessel you fill it completely with water and then force a little more water in. There must not be any air in the tank or it could explode if the tank fails. If the tank fails when full of water the pressure drops quickly and is just a lot of water to clean up.
My plan is to stand the tank on end and strap it to a tree as it will weigh 400+ pounds when full of water. The tank has 3. 1/4" openings one to let air in on the top of the tank, one to let air out in one end of the tank and the drain in the bottom of the tank. With the tank standing on end the drain opening is where I will install a zikr fitting to force my water in. On what is the air in fitting I will install a 200psi or higher water pressure gauge. The air out fitting will be on the top of the tank and will be for filling the tank with water and to bled out the air in the tank. Once the tank is fill I will plug the fitting on top and force water into the tank with a grease gun.
Then you leave it for about an hour under pressure if it holds it's good to go.
Oh my. I am sure there is no way I can convince you that a shade-tree-DIY-hydrostatic test on a discarded tank, with an unknown history, with the intention of depending on the results and actually using the pressure vessel, is one of the most potentially dangerous ideas I've read in here....so I won't bother trying.
I will offer two suggestions:
1) At least do a preliminary visual internal inspection with a bore-scope. Flushing it with dish soap might have cleaned any loose stuff, but as far as assessing internal condition......???? If I want to dive into unknown water, I always check for rocks and depth first.
2) There is a reason that all official and competently done HTs are done in a closed system. In addition to adding a safety buffer in case of failure....it also allows the determination of a pre- and post- dimension so that any change in volume...transient AND permanent can be logged. At the least, consider submerging it in a tank of water. If one of those fittings lets go under 200+ PSI it won't matter if it is air pressure or water pressure if you are in the way.