116v

[Jorge_Stolfi]

How can you tell it's water and not some other kind of liquid?

I can't, really... but any other liquid would probably leave a more visible residue. (Except perhaps "acqua vitae", which I gather was one of the Alchemists favorite experiments...) 

Water would just push out any dirt and loose ink from the page to the edges of the stain, but then evaporate without leaving any residue.

Rain is also a likely source for the liquid.  But of course not the only one.  (For instance, there is a set of greenish stains at the top of many folios that may have been caused by the water where the Painter cleaned his brush. And there is the "coffee" stain on f93r.)

All the best, --jorge