I think the white patches you are referring to are most commonly known as either dysplasia or leukoplakia. These white (sometimes red) patches do not always turn into oral cancer but sometimes as time goes by, they will. There are many other non cancerous patches that can happen in a healthy person's mouth. Because of this, its important to get any sore or patch checked by a professional (qualified ENT) if it has not healed on its own in 2 or 3 weeks.

Patients are all different. Some will experience pain while others have no pain or any symptoms at all.

As far as dentist go, not all dentists are familiar with identifying or treating oral cancer or patients with oral cancer. Many dentists have never even seen an oral cancer tumor and do not know what to look for. This is due to not always continuing their education. Often dentists focus only on the teeth ignoring anything else in the mouth. This is why a dentist is not the best place to go for an abnormality in your mouth. Its best to seek out a qualified ENT who specializes in oral cancer. Always better to be safe than sorry.

Best wishes!