Sometimes you work on the same thing for so long, or you have been doing it for so many years, you just have to laugh at yourself when you get that fraction of a second “Oh no! Which one is the replacement part?!” panic, because you took it out of the static bag, set it down, and had to go do something else for a few minutes.
Then you remember that you taught yourself to always leave at least one cable wrapped up on the new part, make a mark with a Sharpie in a spot that won’t be visible after re-assembly, or one of your other habits.
Luckily, in the case of most auto parts, the one with the most grease/dirt/rust is usually the older part, unless you make the trip to the junk yard, and even then, you’ll often make a clean spot or two removing the part.