First, look for the red color and tall crest with black on the face. These characteristics are typically sufficient to identify the Northern Cardinal.
During the winter, or with juvenile forms, the Northern Cardinal loses the bright red color, and instead becomes more muted. While not as immediately recognizable, the bird call can actually be used to help recognize when the bird is either too far away to see other unique identifiers, or when the colors are muted. See the video here for a sample of a Northern Cardinal call.