“How do you tell if it's a stain or cavity?”

Well, one indication would be to look closely at the surface texture of the dark spot. If the enamel is smooth and the contour of the tooth is still normally shaped, it is possibly a surface stain. If the enamel is pitted or "moth-eaten" in appearance in the stained area, it is more likely to be decay. If the stain is right along the gum line, it could even be the stained tarter build-up if you have not had a professional dental cleaning in a while. You would need to see a dentist for an exam and X-rays to find out with certainty. 

Need to schedule an appointment with your dentist to check it out.

A stain would not be sensitive to hot, cold, or pressure. A cavity may. Best way, however, is getting an X-ray image and consulting your dentist.

A dentist that uses any of the following devices can tell the difference. Devices to use:

-Dexis Carivu
-ECD electronic cavity detector (is the newest way to follow this up because it is unaffected by stain)

Good Luck!

MDSDDS