How do you tell the difference between a canker sore and oral cancer?

Time and/or Biopsy: A canker sore wil go away in a short period of time (usually days). Oral cancer won't. Only definite way to know is with a biopsy.

May need biopsy: Canker sores are usually superficial painful ulcers that tend to heal in a matter of days. Cancer, among other presentations, is a non-healing ulcer, usually painless, more apt to bleed with induration of affected area. Some times it may take a biopsy and examination of the tissue by a pathologist to settle the diagnosis.