You may not be able to. My micron chips had very close serial numbers (2 apart). If you are just trying to use dual channel, figuring this out really doesn't matter, yourdual channel will still work.
If you RAM chips do not have stickers on them indentifying them, you may want to try a freeware program called PC Wizard 2004; It will tell you the part numbers, as well as serial numbers of your installed RAM chips, this may point you in the right direction.