How to display my heart rate on the Samsung "Android Wear" Smartwatch? I have a Samsung Gear Live Smartwatch that says it has a heart rate sensor. 

How can I see the output of this sensor?
There are two applications that come preloaded on the device which can do this for you, "Fit" and "Heart Rate".  You can start these apps by saying "Ok Google" or tapping the screen to bring up search, followed by the command "Start Fit" or "Start Heart Rate" which launch the corresponding app.  Heart Rate's sole purpose is to show this information, so the button to start monitoring will be available immediately.  Fit also tracks other fitness data, such as steps you've traveled, so you'll need to swipe to the left once to get to the heart rate section and expose the button which will start the reading. Both of these apps will request that you stay still for 10-15 seconds as a reading is taken, and it is important to note that they only measure your heart rate when you ask for it, there's not a passive reading being taken.

As a shortcut, you can just tell Google to "Show me my heart rate" to skip the "Start Fit" or "Start Heart Rate" commands as well as the button press to start the reading.  This will automatically launch the default heart rate measuring app and start taking a reading.  You can configure which app does this by default by using the Android Wear app on your connected device.