^^^^ What StarkCity posted works. Mine doesn't go "ping", but more like "click" (coincidentally enough, it sounds like a single 'tic' from a Geiger counter). The watch also made noise whether I switched Silent Mode on or off via the app, even though my watch is set to silent mode.
(adding on) This trick looks easy enough to add as another option in a future version of the iPhone app. The drawback is, there's no way to give the AW's location any more precisely than "within BT range" because the AW contains no GPS or LTE chips of its own. Maybe it can be pinged if it's on a known Wifi network, but the overall capability of a "Find My Apple Watch" button is still limited compared to Find My iPhone. Such a function would need a disclaimer.