How can I tell whether a cavity has reached the nerve?

If pain from something hot lasts a long time, more than sensitivity to cold does, or if the tooth throbs even without any stimulus, the cavity has probably reached the nerve (pulp). By stage: 1) brief sensitivity to cold that disappears quickly - an early to moderate cavity, 2) pain from something hot lasting tens of seconds or more - nerve involvement is suspected, and 3) throbbing at rest that is worse at night - pulpitis is likely. An accurate diagnosis requires an X-ray, so once pain begins, please come in without delay.