How to compare footprints of SMD?
It completely depends on how the footprint for the 0603 part was designed.  Although there are recommended sizes for all of the solder pads for each component, some PCB designers deviate from those standards for particular reasons.  In particular, the pads are often elongated towards the middle of the component precisely to allow a smaller component to be used on that footprint.

This is often done during the prototyping stage to allow maximum flexibility when working with components that are on hand.

That said: you can often make a 0402 component work on a 0603 footprint IF you are placing and soldering the component by hand.  I would not expect that an automated Pick and Place machine would be able to do this.