Once again - if you go to the website I suggest above . . .
http://rvtravel.com/campground-power-pedestal-testing-part-2-with-reader-poll/ . . .
You will read about how the Corp of Engineers campgrounds do not allow people to test outlets with meters. There are comments showing what people think about that. But the answer is simple and I have been posting about it for years - the Prime Products device for testing not only polarity and ground but gives a voltage readout - and is now available on Amazon.
As mentioned in the article - the COE does not want people sticking probes into outlets because bad things can happen when those who are not at least somewhat knowledgeable do this. Sparks can arc and other things happen. Arc can burn the contacts in the outlets making them unusable for others. On that there is also the long standing rule for plugging into or unplugging from a 50 amp pedestal - TURN IT OFF - the reason being the possibility of arc and burning contacts - as well as the prongs on your plug. I turn any and all pedestals off before plugging in if that option exists - it is not always an option - but arcing is the reason.