Magnification isn't a lens specific parameter, it only depends on the object and image distance (do and di) ratio. Basically a lens can be specified by it's focal length. Lenses can be asymmetrical, multi lense optics mostly are, thus the optical center plane isn't necessary identical with it's mechanical center. This should be considered when determing focal length.
The most simple method to determine focal length is to focus the image of a distant object e.g. on a piece of paper and measure di. With do >> f, di is very near to f. By turning the lens, you can check for asymmetrical center plane.
The method works only for positive or converging lenses, also called convex lenses according to their shape. Camera lenses always are of this type.