Are white sores on the underside of the tongue usually cancerous? how could one tell if they are cancerous?

Tongue: If you're 19 and without significant risk factors, it's not likely to be cancer.The only way to know for sure is to do a tongue biopsy. White spots aren't always cancer and can represent a broad variety of benign conditions. You should see a dentist or oral surgeon in consultation to determine what's best for you.

Generally: White lesions are generally not a problem. Your dentist can perform a simple in office test if you are still worried after he or she checks the area. The test is called vizalite and you swish with raspberry flavored vinegar then shine the light on the area to determine if a biopsy is required.