Monitoring Electricity and Surge protection - Tools - he-ruide
A Circuit analyzer (yellow unit)  allows you to check the polarity and ground of the CG post before you plug in your PUP.  I've attached it to a 30 Amp male / 15 amp female adapter (black unit) so I can plug it into the 30 Amp outlet on the post.

A Circuit analyzer (yellow unit) allows you to check the polarity and ground of the CG post before you plug in your PUP. I've attached it to a 30 Amp male / 15 amp female adapter (black unit) so I can plug it into the 30 Amp outlet on the post.