leeneia,

You can't try bouncing the light off the mirror into the guitar. It doesn't work very well that way, it can be done but it isn't easy.
Use a small pocket lifgt like a mag lite or a cheap purse flash lite and put it in the guitar and then use the dental mirror or a compact mirror.

OR

Pull the strap pin, if it has one, out of the tail of the guitar and look through there. A good guitar will allow this, A cheap, older style, or even a good classical may not.

Good luck,

Don