How accurate are dental X-rays in revealing the location (distal, lingual, mesial) of an active tooth decay?

Answer 1

It is a 2 dimensional picture of a three dimensional tooth.

Hence it is limited in diagnostic power. However it is a great tool when combined with a proper clinical exam.

Mesial and distal caries is usually easily discernable from the radiograph. Buccal and Lingual is easily seen clinically in the mouth.