He'll check the inside with a bright light for corrosion or contamination. He'll check the threads with a dentist's mirror for cracking and galling. He'll check the valve, the burst disc and the O-rings. Unfortunately, many inspectors are not trained and don't really know what to look for. Look for a PSI certificate.
PSI is the only full-service organization training scuba tank inspectors.
About half of all tank inspectors are PSI certified. Answers should be clear and confident. Leave a sticker on your tank. Hydro tests, required every five years, include a visual inspection. Then the tank is pressurized with water, usually to five-thirds the service pressure.