First, look at our current model line to see if it is actually still a model we sell. Even if it is not a current model, you’ll be able to identify its type: reflector, refractor, Schmidt-Cassegrain telescope (SCT) , Maksutov.
Second, go to the Telescope Bluebook website. Click on the tab for either Reflector, Refractor, SCTs, Other Designs. The next page will give major brands of these types of telescopes. Click on Celestron. The final pages will give a brief history that will allow you (in many cases) to pin down what Celestron make you have and approximately when it was made.
Look at Michael Swanson’s book, The NexStar User’s Guide, for Celestron GOTO telescopes you know were made in 1999 or later.
Many older telescopes have manuals and information online on the Celestron website. Go to this link for downloads archives. Click on the Manuals link and then telescopes. From here, drill down until you find the manual or model you are seeking. They are PDFs and can be readily downloaded and printed from your computer.