Cavities, or tooth decay, can appear as dark spots or holes on the surface of a tooth. They may be brown, black, or white. In the early stages, cavities may be difficult to detect, but as they progress, they can become more visible. Other signs of cavities include tooth sensitivity, pain, or discomfort while eating or drinking. Regular dental check-ups and practicing good oral hygiene, such as brushing and flossing, are crucial in preventing and detecting cavities early.