I can use my watch to ping my phone, but how can i use my phone to ping my watch

Actually, there is a way for you to do this so long as you have Find My iPhone activated in your iCloud settings. If so, do the following:

1. On your iPhone launch the Find iPhone app
2. In the bottom half of the screen you should see all of your devices, including your Apple Watch
3. Tap on the Apple Watch listing
4. Now at the bottom centre of the next screen you’ll see the word "Actions" - tap on this
5. Now tap on the Play Sound option
6. Your Apple Watch will now vibrate and play a sound until you dismiss it

NOTE: You can also do this from other Apple devices, or from logging into iCloud from a web browser on any computer.

For more information, refer to If your Apple Watch is lost or stolen on Apple's website.