In the past, I explained a method of ringing your phone using from your Smartwatch using Tasker.
Yesterday, Google made my whole post obsolete. And I couldn't be happier.
Now, you can tell your watch to your phone without using Tasker. According to Google, all you have to do is say "Ok Google. Start. Find my phone". Personally, this did not work for me. It instead brought up the option of opening the Google blog post I described above.
Instead, I use "Ok Google. Start app find my phone", and it seems to work nearly every time. Of course, if you're using Android Wear Mini Launcher, you can just swipe right across the top of the watch one time and find the "Find my phone" app directly.
Launching the app will cause your phone to ring at full volume. Assuming your phone is close enough to be connected to your watch, this is an incredibly easy way to find it. Especially useful for finding that phone buried under the mountain of paperwork on your desk.