How do you know when you have cancer in your mouth from tobacco?

Suspicious spot. The only way to know for sure that you have cancer is with a biopsy. If you have a "lesion" (which can be a pimple, ulcer, crater, redness, thickening, etc) which doesn't go away, it should be evaluated by a health care professional. Dentists and oral surgeons should be able to evaluate, but the best evaluation would come from a head and neck surgeon (ent). Risk is high if you chew or smoke.
Check your mouth. Tobacco use, including smokeless tobacco, is certainly a risk factor. Most patients present at over age 50. There may be few or no symptoms. Mouth cancers can present as non healing ulcers that can be painful. Rarely, there can be painless lymph node enlargement. Patients are staged by size of tumor and lymph node involvement.
Changes in mucosa. White patches, ulcer, indurated/hard area, bleeding, may suggest cancer. Diagnosis may require biopsy and examination of tissue by a pathologist.