I have just brought this compressor, which is the best wsy to test it, it looks fairly clean

[leoparddog]

 

[Seadog]

Remove the drain plug to see if there's liquid in there. It'll most likely be an oil/water emulsion.
The correct way to test is hydrostatically, that is with pressurised water to a pressure of 16.5 bar (approximately 250psi).
A plumber's test pump is ideal.

 

[Sam0117]

Hydro test it, fill it with water and have a pressure gauge on the tank, you can either adapt a jet wash to add more water to pressurise the tank or like mentioned about a hand pump for plumbers, as long as it holds the 16.5 bar for 30 minutes it’s all good

 

[brightspark]

just turn it over and tap it along the bottom with a a ball pein hammer if it dents its on the way out but hardly likely.if they ever leak through rust they just pinhole 1st

 

[bourbon]

What make is it?