Red and black wires

IIRC the only place I've seen non-polarized 120V plugs or receptacles recently is on miniature holiday lights. Maybe that's to keep you from using the receptacle on the end of the string for anything but another light string.

I have some old stuff with switches or lamp holders in them that have nonpolarized plugs. That is pretty much worst case. I also found a couple of 18ga non-polarized extension cords in my mom's stuff. I put them in with the christmas stuff, just for those cheap lights with 20-22 ga wire.

Cell phone/iPad chargers

... or anything that is not polarity sensitive. The cheap christmas lights get away with it because there is no switches or lamp holders with a "shell". The chargers are polarity agnostic too since they simply feed an isolation transformer or a switching power supply. Most electronics are the same way these days.

Are you certain?

What sucks is when you are up on a ladder attempting to connect one string to another only to find that the plug from one set *is* polarized but the socket is not.