The rotary crown button functions introduced in Android Wear 2.0 have made their way into the Wear Simulator of the latest Canary edition of Android Studio. This means that developers no longer need a physical Android Wear 2.0 device around to test the behavior of a function that they've mapped to the rotary dial in their Wear app, so long as they're willing to hop over to the Canary build of Android Studio or have already been using it. The tester is virtual of course, so it does not account for hardware quirks, but should be sufficient to give developers a decent idea of what users of their app will see when they turn the rotary crown button.
The tester for rotary crown button functions found in the newest Canary version of Android Studio is fairly easy to use. All a developer has to do is grab a virtual handle with their mouse, or their computer's touch screen if applicable, and give it a crank. That handle is found in a screen that shows the side of a virtual Android Wear 2.0 device. Developers can crank the button in either direction, and the result will be shown on a separate screen that shows the front face of the device in real time, making testing of fine functions about as easy as is possible for such a process.
Google recommends that developers use the rotary dial to scroll the view in their app, which is likely one of the easier implementations out there. As with the Material Design guidelines and other examples set forth by Google, this will most likely end up being one of the most popular ways to use the dial. Other uses are not outright disabled, of course. Like any other control element of an Android Wear device available to a user, developers can mess around with the function of the rotary crown button as they please. This means that developers who want to do things like have the user play a game, navigate music tracks, adjust settings, or other such functions can do so, and use the new tester to ensure the behavior works as intended.