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.

2025: I found this note in This is True:

Automotive engineer Jim Moylan in 1986 suggested to his bosses at Ford that fuel gauges should indicate what side the filler was on to help drivers. Ford didn’t patent the idea, presumably on purpose, and it spread across the industry. The indicator is now called the Moylan Arrow.