How do you know if dentist is right you need root canal?

You can get a second opinion, you can ask the dentist for the facts including any x-ray evidence. The first time I got a root canal over 40 years ago I am certain beyond doubt that I needed it. The dentist began drilling and I pointed out that he did not administer novacane. He said "Don't worry; you don't need it." It was a front tooth; I felt nothing. However, I don't recommend this method of proof in all cases. A root canal may be indicated even if there is some life left in the nerves.