After checking with user control panel, or checking with server app, you considered that there is no sync anymore (you saw the last sync is too old - 1 day ago for example).
If you are keeping the target device now, then you can check if the app still is being installed on the phone:
1. If you have hidden the app, try to show it. If you are able to show the app, it still works.
In order to show or hide the icon, make a call (dial) with an access code.
2. If you can see the icon of the app, 99% of the app still works.