How can I tell whether I have esophagitis or esophageal cancer?

Biopsy. Biopsy during fiberoptic endoscopy is the cold standard to distinguish between esophagitis and esophageal cancer.
Esophagoscopy. Esophagitis can be symptomatic or silent, esophageal cancer rarely causes symptoms until it is bulky enough to cause swallowing problems. Esophagoscopy can differentiate between the two with biopsies needed to exclude cancer.

Related Questions

What to do if I have esophagitis, am I at risk of having esophageal cancer?

Yes. Barrett, s esophagus and esophagitis are significant risks. In others with hiatal hernia, or strictures with the reflux and irritation can be followed by irritated cells and malignancy.
Possibly, unlikely. Esophagitis is common. Esophageal cancer, while it is on the rise, is relatively uncommon. There is a type of esophagitis called "barrett's esophagitis" which is associated with esophageal cancer though even in this setting the risks are lower than what we once thought. Typical testing would include an egd (endoscopy) with multiple biopsies to characterize the esophagitis.

How likely would it be to hav esophageal cancer only 8 months after an egd that showed mild chronic esophagitis?

Low likelyhood. The period of time is probably too short to develop even premalignant conditions in the epithelium of the esophagus. However, if you have symptoms that concern you, discuss this with your primary care provider or with the gastroenterologist who did the procedure. Do you have chest pain, post-meal pain, hoarseness, bitter taste, sore throat, difficulty swallowing. Could be reflux disease.
Unlikely. It is very unlikely for a person to develop esophageal cancer in 8 months. It is possible (but very rare) for an egd to miss a very small cancer. It is extremely unlikely (almost impossible to imagine) a patient developing a cancer that would be large enough to cause pain or cough in 8 months.

Is it possible to have esophageal cancer 8 months after a egd that showed only mild chronic esophagitis cause by reflux?

Possible, unlikely. The cancer type of the esophagus that occurs with reflux is adenocarcinoma. Esophageal adenocarinoma usually in a setting of gerd and barrets esophagus, age 60's, m>f. If your prior endoscopy did not show barrets or dysplasia 8 months ago, it is unlikely. However, best to have further evaluation from your gastroenterologist. Best wishes.
Yes. Yes, it is possible. However, I encourage you to speak with your family physician and the gastroenterologist and discuss your concerns and/or symptoms.