This app lets you monitor your iPhone's battery from your Apple Watch12
An app for the Apple Watch called Battery Monitor, created by a gentleman named Tom Etminan, will allow you to check your iPhone's battery life after enabling it in Glances. To do that, after installing the Battery Monitor on your phone, go to the Apple Watch app on your iPhone and from there tap on My Watch > Glances, add Battery Monitor. The app will warn you when the battery on your iPhone falls below a certain percentage, and will also tell you when the cell is fully charged. The animated ring employed by the app uses different colors depending on how much life remains. This way you can get the information you want with a quick glimpse.
For now, Battery Monitor is a free download from the Apple App Store. However, the developer hints that he might be adding new features to the app in the near future which could be accompanied by a price hike. One question is still caught on the tongue of the Apple faithful. Why didn't Cupertino come up with this first?
source: BatteryMonitor (iOS) via RedmondPie