You can figure out foreshortening by using the rules of perspective. It would be too complex to describe the rules of perspective here, but in a nutshell...
The basic idea of linear perspective is that objects in space look smaller the farther away they get from the viewer. If you learn to draw a box using linear perspective you will be able to draw boxes that appear to be in a three dimensional space. Anything can be put into a box. The parts of a thing can be broken up into smaller boxes. Even figures and the parts of a figure can be placed in boxes. The more you can master the rules of perspective the easier it will be for you to draw things in a 3D space. You will understand how and why things look foreshortened.
