so if I run an app on my iPhone, which saves data to a file

what's the best way to get that file out of my iphone and examine it on my computer ?

You can FTP it to a file server
Set up a web page that can take POST requests and send it to a server that way.
Once you subscribe to Corona, the latest daily build supports email with attachments.

In theory, you can put the right key's in your build.settings plist area that would turn on the ability for iTunes to be able to see the system.DocumentsDirectory, though when I tried that, it didn't work. Doesn't mean I didn't make a mistake.

And in theory, if you're running Xcode's organizer while tethered, you should be able to access the apps sandbox folder.

use xcode organiser... it's no problem.
just navigate to the app in the list of apps on your device and click the arrow next to it.
Then it will copy the sandbox folder to your desktop.

@ojnab, that will work if the poster has access to the device and can tether it. But if the goal is to get data from someone else's device running the app, one of the other options will need to be implemented.

perfect thanks all!