How can the opticians find out the eye power by looking at old spectacles?

Neutralization. A lensometer is a device that reads the power in a lens by meutrilizing its power.
Lensometer. This is a machine that can read the prescription of the lenses in the spectacle.
Eyeglass power. While an optician can find the power to make eyeglasses from by copying an old prescription, this number may not be accurate and it would be useful to update your value by having an ophthalmologist redo your refraction before you spend money on eyeglasses.