In a light switch, how can I tell if the black wire is really hot and the white wire is really neutral?
In a perfect world, black is hot and white is neutral. Unfortunately,that's not always true. I have seen houses wired opposite, against code. I've seen part of the house wired opposite. What you need to do is get a meter, not a wonder stick, and test your black group for voltage to ground. You should get 120v +-, then test white group to ground and you should get 0. If you get 0 when testing the black and 120V when testing white, then it's backwards and you or a pro should figure out why. It could be wired correctly and maybe whoever installed this switch decided to switch the neutral instead of the hot. Once you determine hot and neutral, you can think about installing your outlet.