If you have seen the movie A Bronx Tale, then you already know exactly what I'm talking about. This rule, is a classic rule of thumb and in the movie, even though they are referring to the way the woman reciprocates, to me it is a two way test.

Test for Man: If the man goes around to the passenger door side of the vehicle and opens your door for you to get in, he is definitely a keeper.

Test for Woman: After a man opens your door, you should reach over and unlock his door.