The easiest way to show a card types key on a board, is to enable the Card Legend Power-Up.
Another way to peek at the available card types is to navigate to Settings - Card Type. This is also where you can change the existing card types, or add more.
If you like, you can also set up a card type legend through the Lane Policy field. To try it, go to Settings - Board editor, then edit the column in which you want the card legend to be displayed, and enter your information as Lane Policy / Description field. You will be able to view it by hovering over the columns information button:
Finally, you can glance at what types of cards there are, by right-clicking any task and testing the Change Type option. With or without changing the type, you will see what card types there are to choose from.