Most often in the early stages it goes unnoticed. It's usually painless and may show little physical tissue changes. It may appear as a white or red patch, a lump or a small ulcer which looks like a common canker sore. Because there are so many benign tissue changes that occur normally in your mouth, it is important to have any sore or discolored area of your mouth, which does not heal within 14 days, looked at by a professional. Other symptoms include pain or difficulty in swallowing, speaking, or chewing, any wart like masses, hoarseness which lasts for a long time, or any numbness in the oral or facial region.
Other than the lips, the most common areas for oral cancer to develop are on the tongue and the floor of the mouth. Individuals that use chewing tobacco, are likely to have them develop in the cheek area. In the US, cancers of the hard palate are uncommon.
If your dentist or doctor decides that an area is suspicious, the only way to know for sure is to do a biopsy of the area. It is important to have a firm diagnosis as early as possible. It is possible that your general dentist or medical doctor, may refer you to a specialist to have the biopsy performed.