I came across "Zone System Analyzer" a few months back at just $.99

All the other apps that had "Zone System" in the title were more fancy-photo note taking apps.
Not that's a bad thing, but didn't provide any scene analysis.
With this app, you take a picture with the iPhone (iPhone 5 is best for a lot of reasons)
Then after opening the app, you select the image from the Camera Roll, etc
A zone scale across the top lets you select the zone you are interested in and after selecting it,
will highlight that zone's values in the image.
Obviously how you took the image, the iPhones exposure metering, dynamic range, etc will have an influence.

The nice thing the app now has a zone threshold setting option for each zone so you can fine tune it.
I know, the purists say that you should "know" the zones instinctively, but I'm not there yet.
I find it handy to keep things in the ball-park for scene analysis.
With the thresholds settings, you can tailor it to your shooting preferences.
Here is the link:
https://itunes.apple.com/us/app/zone...646725291?mt=8

Enjoy