I bought a piece of equipment. The requirements are:
115/230V 50/60 Hz (single-phase)

I measured the voltages and this is what I measured:
Line to neutral - about 120VAC
Line to ground - about 120VAC
Ground to neutral - less than 1VAC

How do I know if this is single phase?

 

I bought a piece of equipment. The requirements are:
115/230V 50/60 Hz (single-phase)

I measured the voltages and this is what I measured:
Line to neutral - about 120VAC
Line to ground - about 120VAC
Ground to neutral - less than 1VAC

How do I know if this is single phase?

A standard 120V outlet is single phase. The ground pin is just a ground pin for emergency, chassis faults, not a current carrying by design.
Therefore, a three-phase system would need four pins (hot, hot, hot and ground). You would have equal voltages across all three hot combinations. Those three equal voltages would most likely NOT be 120VAC in the US.

 

Just to add, two current carrying AC conductors are 1phase.
Even when 2 phases of a 3 phase supply is used it is a single phase supply.
Max.