When is the application loaded onto PLC out-of-date?

Best place in Neuron Power Engineer to identify an out-of-date application is within the Instances view:

Compare the fingerprints displayed as values for the IEC-resource and for Code identification. If they are different, usually the version on the PLC differs from the one in Neuron Power Engineer.
Possible causes for different fingerprints are listed under "When is the fingerprint for an application not changed and when is it changed?".

Modifications outside the application make "Refreshing" necessary.

If you edit/create →resources outside of Neuron Power Engineer, refresh the view in the project explorer afterwards!
This triggers Neuron Power Engineer to refresh the view of the resources and its children and the fingerprint for the application created in Neuron Power Engineer.
Alternative for refreshing: Open the edited/created resource in Neuron Power Engineer.