The 20 - 30 or so volts you are reading, with the main open, is probably due to the impedence of your DMM. The easiest way to prove this, is to test voltage with a wiggie in parralel with your DMM meter leads. (The wiggie will provide enough load in the circuit to eliminate false reading.) As for testing the meter socket, most utiities provide their installers with meter socket testers, a unit that plugs into the socket. It contains 30 amp fuses connected line to load, that will blow is more than 30 amps of load is present on any phase or if a short is detected. & two sets of neons light to indicate safe or unsafe conditions exist. These units are invaluable to the metermen, because the only true way of proving a meter socket is safe is to test all phases in the socket at the same time.
And it goes without saying that all Personal Protective Equipment must be worn when working in any energized meter socket or panel (we require 1000v rubber gloves with leather protectors, eye protection, nomex uniforms & boots be worn.)