How do you know for sure if its a canker sore? could it just be a yellow open sore? without a red border?

See your dentist : Your dentist will tell you for sure. Could be a canker, but may be a parulis--or opening for a sinus tract (abscess drainage port). Both will exhibit inflammation (redness, pain, swelling).

See your dentist: Patients who get canker sores frequently, know what they look like. If you want to be sure, you will have to see a dentist.