Sometimes it can be obvious, even from images, which side of a leaf is the hair side (the outside of the animal’s skin) and which side is the flesh side (the inside). Here is an example from Bodleian Library, MS Bodl. 565 (The Itineraries of William Wey).[1] Folio 11r, or the left, is the hair side; folio 11v is the flesh side.
MS Bodl. 565, f. 11r (hair side). Photo Mary Boyle, courtesy ©Bodleian Libraries, University of Oxford
MS Bodl. 565, f. 11v (flesh side). Photo Mary Boyle, courtesy Bodleian Libraries, University of Oxford.
Ask yourself how we know, and what factors might make it more difficult to tell.
Mary Boyle, University of Oxford
[1] Photos Mary Boyle, courtesy Bodleian Library.
Teaching the Codex 