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). Folio 11r, or the left, is the hair side; folio 11v is the flesh side.
Ask yourself how we know, and what factors might make it more difficult to tell.
Mary Boyle, University of Oxford
 Photos Mary Boyle, courtesy Bodleian Library.