How do I enable a dirty check on a modify button, when a field value has changed? E.g. If a value on a specific field has changed, how do I add a dirty check to warn users of unsaved changes.?

Only on a specific field? I don't know of an easy way.
What I usually do is:
Create ActLink on Window loaded, diaply or open (depend how you open the form) on runlevel 999
Do Run Process
SET-CHANGE-FLAG 0
This is to clear the dirty flag after everything has been set
Now as soon as the user changes anything, this will change to 1
Then you can use GET-CHANGE-FLAG to check if it has changed :-)
Hope this helps

P.S. Cant remember if SET-CHANGE-FLAG takes string ("1") or integer (1)

Integer