Correct polarity of outlets from breaker
Inverted receptacles are a thing. Sometimes it makes sense - e.g., I have a single receptacle under my gas cooktop for the igniter plug. It is mounted upside down - the theory is that since that is the only thing plugged in (it is a single receptacle, not a double), might as well minimize one small possible source of problems - plug pulled slightly out where still making contact, if something falls down it will hit ground instead of hot+neutral - not likely, but "why not". What matters is how everything is wired up, not how it is mounted.

This is a very good use for a magic 8-ball (Harper's term) tester:



If the receptacle works in general and the tester shows the correct 2 lights on, 1 light off, then it is wired correctly and you can trust that the wires are what they are supposed to be. Just pigtail off of the existing hot &amp; neutral to connect additional receptacles or other devices.

Just remember, AC polarity is not like DC polarity.

With DC, if you get it reversed, sparks may fly and, in any case, things won't work. (Usually - e.g., put batteries in backward in an LED flashlight and it won't light up; but put them in backward in an incandescent flashlight and it will light up). 

With AC, if you get hot &amp; neutral reversed, everything will be fine until someone gets zapped or your house burns down. Your motors, lights, heaters, etc. can't tell the difference (unless they specifically reference actual ground) but SAFETY DEPENDS ON CORRECT WIRING.