The calculated efl. Flatness on window glass will vary. A very quick but surprisingly sensitive test is to view very distant streetlights or stars reflected in the surface at a grazing angle of 45 degrees or less. The test can be made much better by using binoculars to view the reflected image from the glass surface. If the image remains round when slightly defocussed both inside and outside of the focus, the surface is flat enough for application in a small diameter lens.
Homogeneity is a problem especially with steeper curves.
Viewing through a plane polished sheet usually hides these errors if they are in layers parallel to the surface although viewing distant objects using binoculars through the sheet is more sensitive. Examination of lenses after they are shaped is a far better test but somewhat frustrating. Fine grinding the edges and temporarily fixing on a thin piece of glass with salad oil enables edge examination. Immersing unpolished blocks in a fluid such as kerosene is useful although a liquid of refractive index of 1.52 allows better examination. A detailed discussion appears in one of the ATM books.