Hi C.,
Take a photo of each pair of shoes and tape them to the front of the boxes - that way you'll know exactly what's inside!
4x6 digital prints are pretty inexpensive (I think they're 9 cents at Walmart). Even if you have to buy a disposable film camera and get the photos developed, it'll be well worth it to save you a lot of headache in the future.
Make sure you zoom in (or move in closer) and take the photos such that the shoes take up most of the picture. You'll want the shoes to be as visible as possible so you can still see the photo on the box at the very top of the stack.
Good luck with the organization!
A.