The only way to properly test it is hook up a water line to the boiler with a ball valve, install a pressure gauge, the hot water coil, and then cap off all the other openings except the top, leave that loose so all the air can escape when filling it. Once it's full to the point where the water is streaming out of the top plug, cap it off and bring the boiler pressure up to 30psi. Shut off the water and close the ball valve, check and make sure the gauge reads 30psi, dry off the boiler completely and go do something else for a while. If the gauge reads 30psi still then it's a good one! If it leaks it should be obvious. I gambled with mine, I hooked it all up and then pressure tested it. It's a steel boiler so it isn't going to crack like cast iron and I doubt it will ever rust through but it can't hurt to test it. As far as sandblasting it that's probably a great idea, you don't want all that rust in the system. Rust in the firebox will take care of itself once you light a fire
