I found out a friend of mine didn’t know this simple clue about which side of a car the gas cap is on (especially useful if you borrow someone else’s car or are using a rental): In newer cars, the dashboard has a symbol of a gas pump, with a triangle pointing to the side of the car where the gas cap is.

Some searching of the Internet suggests that the charging port for electric cars depends on the manufacturer, and I did not find any indicator for these.