What do they look for in a dental xray?

Cavities, bone loss. Depending on the angle or position of the radiograph (x-ray), dentists are looking for cavities, bone loss (gum disease), root resorption, infections, or other abnormalities in the jaw.
Abnormal features. The simple answer is decay, open edges of fillings or crowns, bone loss etc. However, what a good practitioner looks for are things that appear different from what would be normally expected. Such as a shadow on otherwise solid tooth structure. A tooth or a dense white opacity where there maybe no explanation.
Variation. As with visual examination, lab tests, etc, what is looked for is variation from the norm. Changes in tooth structure, bone morphology, soft-tissue outline, impactions, missing teeth, extra teeth, etc. The list goes on forever. Dentists take only those x-rays required for thoroughness. Dental x-rays are extremely low dosage. If you have questions, ask your dentist.
X-ray diagnosis. Cavities, broken fillings, fractured teeth, bone loss, cysts, malformed teeth, impacted teeth, infections, neoplasms, fit of existing restorations such as filling, crowns, inlays, veneers, health of nerve canals, tooth and root morphology, proper number of teeth, development of teeth and roots, tooth angulation, impacted teeth, extra teeth, missing teeth, and a lot more.
Dental disease. The dental x-ray images reveal hard surface diseases and abnormal findings in bone and teeth. Dds/dmd are trained in head and neck anatomy. Also dental anatomy. The images show caries (decay) bone loss and bone diseases asa well as if restorations (fillings/caps) have proper fit. Digital x-rays are far superior in aiding dds in diagnosis of diseases.
Shadows/bright spots. Dental x-rays show contrast in the density of calcified human tissues. Where dentist placed treatments are present, they often appear as bright spots. Darker shadows, both in bone and teeth are often signs of an on healthy situation. This happens to either loss of bone or tooth calcification. The height of the bone around the teeth is a critical measurement of stability and health.