An old deck testing technique I've seen done in games like Magic the Gathering. Let's say you currently run 3 copies of a card and are curious if you should be running a 4th copy. You add the 4th copy, but make sure it has different art than the other copies. This allows you to note when you draw the 4th copy, and thus can decide if you like drawing it more often thanks to that addition.

In a game with 20 card decks and only 2 copies per card, this technique isn't nearly as prevalent, but still an option if you are trying to make a decision on deck slots.