The Connecticut Insurance Department requires insurance companies use at least two of their commissioner-approved car pricing guides to determine how much your car is worth. They prefer one of them be the NADA Used Car Guide. The other approved pricing guides have links here.
The insurance company will then average the listed retail values to come up with their offer your car.
They must send you their itemized valuation report or appraisal explaining how they arrived at their total loss figure. Experienced accident attorneys can often, but not always, get a better offer for you.