The upper right-hand corner of the note-card contains the source information.
Usually the name of the author and/or the page number where the information on the card was found goes in the upper right-hand corner of the note-card. Maybe if you're also using source cards, the upper right-hand corner will contain a source number, thus you can reference your source card for the necessary information. The top of the note-card should contain the title with the topic the information will deal with or support.