While you can easily search for subjects (e.g. Biology) on our public marketplace, Brainscape currently only offers the ability to search for specific flashcards using our iOS app. We hope to bring it to Android and the website at some point in the future.
To search for a specific card (from among the Classes in your account), tap on the menu in the top-left of the iOS app's main screen, and enter the search term for the card you are looking for. You'll see a list of all the flashcards that contain that keyword, and you can tap on any of them to see the full details and rate your confidence.