How to Tell the Difference Between Mexican and Regular Shoes
This section assumes that "regular" shoes are those found in typical US/European boxes, which are shoes made in either China, or Hong Kong.  The white shoe is the example of a typical shoe.  The reddish shoe is the Mexican. 
The most obvious differences are shown below.  Other things to note, are that Mexican shoes tend to be shinier, and the colours will be slightly off, they will not match with regular shoes. 
It appears as though the Mexican shoes were original cast from a doll shoe, thus leading to the smaller size and loss of detail. 
Since Mexican dolls seemed to also use "left overs" from Hasbro, I'm guessing some MIB Mexican dolls also may have Hong Kong or Chinese shoes.
Notice the difference in size.  The Mexican shoe is smaller. The sides of the shoes also curve in slightly and tend to be thicker. 
Placed sole to sole, the size difference is obvious. 
You will also notice the heel on the sole is more rounded, less defined as a half-circle the way a regular shoe is. 
 This information was discovered by me on my own, with my Mexican dolls.  So far, only Laura (Badgerprincess) has helped confirm this information- so obviously our resources for this were small.  This could therefore be called a "theory" at this point, not proven fact- feel free to send in any information you have confirming or opposing this theory.