You could attach fittings to the outputs, and put a schrader valve in one of them, you could pressurize it with an air pump. 20-25# should be safe...just keep it below the (probably 40#) of the overpressure relief valve. Keep in mind temperature changes will change the air pressure, but it should hold overnight. You might hear air escaping if it's bad or never be able to build up pressure. If you could get some water in there before you pressurized it, the wetness may be easier to see. Some cracks may not open until things are heated up, which would be much harder to find just sitting there.