Any suggestions besides the obvious pencil or highlighting with colours? I am stumped, this was trickier than I thought. It is for a search function/filter if that helps but a universal symbol would be prefered.
Whatever icon you select is likely to be specific to a given culture, so consistency is likely to be more important than the actual icon.
Things I've seen used ...
A tiny (8x8 pixel) disk shown next to the top right of the field
Colour and Style change - modified fields changed to Bold and Blue
As a side thought: I think highlighting modified fields is a good idea.