I will make testing super easy
First you need 3 items.
15 to 30 adapter, Puck styleNote: you can use a dog bone style or a round puck instead of this shape, none of this matters, Only the plug and socket matter.
I am not a fan of Wal*mart but this is the type of tester to check polarityAnd finally: Something to do the job of the following:
Analog version, very easy to useDigital version, also monitors frequencyAdvanced version digital, does many things, I use one myselfUse the 3 light tester and adapter to decide if the polarity is right
use one of the volt meters to decide if the voltage is right
You can also use a multi-meter for that but alas,, They do not always have probes long enough to make contact these "We have to protect the user" days (old days they did) but the devices I liked to all come with 15 amp plugs.