Shockley Equation, 1N4001 and 1N4003
The 1N4001 - 1N4007 are all the same diodes. http://www.diodes.com/_files/datasheets/ds28002.pdf

Since all the characteristics are the same, the manufacturer will simply select/test the diodes to meet a range of specifications. Those that have higher leakage or break over voltage become lower range diodes. There will be differences in the manufacturers line, but they would invariably want to produce consistent results from a single build process. This plays out in much the same way the as the production of large scale chips such as microprocessors, where the power and clock rating of Individual die will vary with it's position on the silicon wafer.  

You'd be better to measure the high reverse voltage leakage current as a comparison characteristics, but even here it likely won't be possible to identify individual voltage ratings.