First, consider the size of the steaks. Tuna steaks can vary widely in size, so it's important to choose steaks that will fit comfortably on your air fryer.  Second, think about how you want your tuna cooked. Tuna is best when it's served rare or medium-rare, so you'll want to choose steaks that are at least an inch thick.  Finally, take a look at the color of the tuna. The meat should be pinkish-red in color, with a healthy shine. Avoid tuna that looks dull or gray, as this is a sign that the fish is past its prime.