how do i tell if 50 amps is coming through plug.

There is a couple variables why it didn't work then it worked.
I would say the rig's cable or connectors are questionable.
Could be cord body at the pedestal end having one partially open or completely open when wiggled.
If the rig has a twist lock connector to attach shore power cord, same as above.
If the rig has a retractable cord reel the slip rings could be burnt, corroded or not making contact.
50 amp shore power is 120/240 four wire. two legs of 120 on different phases, a neutral and a ground.