Simplest method for determining "changes made" on a form/card

[Sivakatirswami]

Without scripting every field, if you have a data entry system with 
multiple fields, and you want to flag the user that he or she made a 
change but did not save before click "next" or "Previous" to go to 
another record... what would be the simplest method to check that the 
card was modified to then prompt the user "Do you want to save your 
changes?"

Sivakatirswami