How to tell whether metal junction box is properly grounded?
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.)