How to tell the difference between a canker sore and oral herpes?

Canker solitary. Herpes are usually in clusters of two or more sores. The area around and under the ulcer is hard, swollen & tender. The edges of the herpetic ulcer are typically indistinct. A canker is usually a solitary ulcer with a discrete uniform edge, round to oval in shape, with a definite bottom - like a crater on the moon. The area around & under the canker is neither swollen nor hard.