Location of the serial number
The serial number is located on the hard back piece of the case. It has a white background with a 11 character / numeric code. It is in different places, depending on the model.
My case does not have a serial number, or I can’t find it
OtterBox has only recently started using serial numbers. The vast majority of cases will not have one. Out of all the cases we used for our OtterBox counterfeit guide, only the Samsung Galaxy S5, and Note 4, and some iPhone 6 cases have the serial number. Older Apple models do not have a number, either do BlackBerrys and older Samsung models.
How to check your serial
There is really no need. I have not seen a single counterfeit OtterBox that uses a serial number. I am sure they will start to once OtterBox has made the serial number common on their cases.
If you still want to check the serial number, here is the link to OtterBox’s website. However at the time of this post, the program returns any entry as true.