For many decades they have been physically polarized, so it is impossible to plug them in the wrong way. Some electrical devices do not have a polarized plug, likely cause with them it does not matter or they are old.Maybe American outlets do. Here in the Netherlands our outlets are not polarized.
My boat has a polarized twist lock 30 amp plug, can only plug in one way. Then on the neutral wire are small neon lamp connected to ground, if it lights up, you know the polarity is reversed.