Andrew ....
Your interesting post -- begs a couple of questions - like - " and then do what with that knowledge ? " - if you find out the data has changed - what then ?
I'm positive you could use setInterval() function depending on what you want to happen with this information...
http://www.w3schools.com/jsref/met_wi...
If the app was truly off line , and closed (not running in the background) you could use a server code routine to review the data - and tnen 'push' notifications to the user's device ....
just a couple of thoughts..... knowing the end game might trigger a different direction though....
Best,
Bruce