I've had a small pin hole on my upper soft palet for 3 years. Today I am breathing in my nose and air comes in my mouth. How could I tell if its cancer?

You can't. But a Dentist can. See your Dentist ASAP, and ask for referral to an Oral Surgeon, or see your Physician and ask for referral to an Ear/Nose/Throat specialist. I'm curious as to how the cause of the hole has gone undiagnosed and untreated for 3 years.
Oro-antral fissure. That's a possibility. You can't possibly tell if it is a cancerous lesion. Please make an appointment with a local oral surgeon to have it examined and treated. You should not leave it alone. http://www.ehow.com/way_5566699_oroantral-fistula-treatment.html http://www.exodontia.info/OroAntralCommunication.html.