Are optometrists able to tell what my contact lens prescription are by just looking at your glasses?

No. Fitting of contact lenses involves more than just the power of your corrective lenses. The corneal curvature, size of your cornea and general health of the anterior surface of your eye are taken into consideration. How your eye tolerates a specific contact lens is determined during the followup phase of your fitting.
No. The glasses prescription is a necessary piece of information to determine a contact lens prescription. But there is more to it than that.