You could get a rough idea from the serial # to tell if it is a early or later realeased NES.  The do have differences though depending on how new they are.  The newer ones had better lockout protection to prevent unlicensed companies from being able to easily make games work.

Then of course some of the different region ones look a little different as well.

Thank you friend, I know i brought this up before and no one had a good answer.  Does anyone know how bunnyboy came up with this info?  Is this just based on him cracking open a ton of Nes's and documenting the versions he observed. 

If anyone here knows exactly when they got their nes please post what your serial # is.  Maybe we can figure out some sort of rhyme and reason to it like a Vin or something.  That'd be cool if the first four digits was the location of manufacturing, and then the next four was region, etc.

Unfortunately I've amassed so many toasters I didn't keep track of which one I originally got back in '88 with no game and a sweet player's guide.