Summarize this article:

All authentic Nike shoes have a tag sewn into them with their size, barcode and model number on it. You will recognize it most easily by the barcode. Look for it inside:  The tongue The heel The arch The model number of your shoes is typically located under the size and above the barcode on the tag. It will be a six digit number followed by a three digit number (Example: AQ3366--601). If you still have the box that your Nike shoes came in, the model number will be printed on them. Look for it on the sticker that also displays the barcode and size.

Summary:
Locate the tag inside your shoe. Find the model number on the tag. Find the model number on the box if the tag is missing.