Manual:$wgShowUpdatedMarker

Details
If watchlisted page was changed after it was visited:
 * history page: new edits are marked with Updatedmarker
 * watchlist: the page title is bolded

Also the watchlist has a message Wlheader-showupdated and a button Enotif reset that marks all entries as "visited".

Dependencies
If $wgEmailAuthentication is false, this setting is ignored.

Styling
For different styling of watchlist entries see w:WP:Customizing watchlists.