Could a shadowy area below a tooth root on an xray be a large abcsess if you have no pain or swelling and should you let the dentist do a root canal with no anesthetic to see if you feel pain?

Root canals can and are done without anesthetic if a tooth is known to be dead. This can usually be determined *before* the root canal is started. To answer your question, if a tooth is dead and asymptomatic, you may be able to have the procedure without anesthetic but the main reason would be to avoid having a shot and hours of numbness, rather than to "see if you feel pain." I would see an endodontic specialist if possible.

It could be, but never let a dentist do a root canal without anesthetic, even if the nerve is removed there is still tissue there which feels pain. I know, I had it done by a dental student. She thought that because the nerve was removed (the week before) I wouldn't feel pain, and her supervisor thought she had given me a local anesthetic.