I do not remember the formula

If it doesn't have a placard or anything to indicate the last time it had a hydrostatic test I would absolutely have one done before pressurizing it. Although, if there's no placard, you really don't know what pressure the tank was intended to hold, and even just 10 or 20 psi can make a BIG difference to the amount of force on the tank.

A welding supply shop might be able to help with the test, if they can't do it they'll still know someone that does because all their tanks need periodic hydrostatic tests.