I have a detached garage that is powered by a circuit breaker in the main house’s breaker box, which is properly grounded. The outlets in the garage have metal junction boxes, fed by metal conduit containing two wires (hot and neutral). There is almost no voltage difference between the neutral wire and the metal box; how can I tell if the box is truly grounded, and not just tied to a common wire somewhere inside the wall?
Properly assembled metal conduit is an approved grounding conductor.
If the conduit feeding the garage from the house is metallic and connected to the box at the garage, to which the conduits are attached, it's grounded. If there's a proper 4-wire feed (or 3 wires and metallic conduit) the neutral at the garage should be isolated (visibly insulated from the breaker box) and the box itself should be grounded, either to a grounding wire or to conduit (and also to local grounding rods, or concrete encased electrodes.)
Unplug everything in the garage and turn off the breakers to it. Remove the neutral wires from the neutral buss in the main panel. Turn on the breakers and in the garage read the voltage from hot to ground in a junction box. You should have the same line voltage as in the panel from hot to ground. I did this once when I had a rusted through conduit feeding a garage and had weird readings to ground.