There is no way to tell if a title is being watched, via the API. This is ubiquitous in the UI so should have some API vector. If a bit expensive it should probably be a non-default prop=info parameter like &inprop=watched returning watched="".
If not expensive, this could be generated (optionally optionally) for all logged-in queries using &titles= and/or a list module?
<Reedy> and we had it setup to work out if the page was currently watched, to set a button to watch/unwatch as appropriate...
<Reedy> So was after a way to move that accross
<Reedy> We pull action=query&prop=info|revisions&intoken=edit&titles=Main%20Page&rvprop=timestamp|user|comment|content currently.. So if it was in there somewhere without pulling more/an extra query, that'd be nice