Here is a web site for
50 amp service and another for
30 amp service.
These show the pins on each connector and describe what should be connected to each.
The 50 amp has two hot legs. Each of those should read approx 115-120 VAC relative to neutral. And they should read (approx) 230-240 VAC relative to each other.
The 30 amp has only one hot leg and should read approx 115-120 VAC relative to neutral.