There is currently no read/reset access to the watchlist timestamp. This timestamp could be used for something like an update marker:
Adding this as an API feature would save editors with large watchlists from having to scan through to the last thing they checked. It would also allow editors to save certain watched items for later evaluation. This would save much time and hassle.
The relevant API calls may be:
Adding a param to edit-watch that sets the time (if blank sets to the current time) and lastchecked as a returned field in the watchlist list would resolve this issue.