Conditional Formatting on Cells with Formulas

I have a large schedule worksheet with calculated dates. The user will sometimes want to type a date over the formula if the schedule changes. I would like that cell to change color using conditional formatting if they override the formula with a date so we can quickly see which cells have been customized. I can't find a function that will return whether cell has a formula in it, however. Any help is greatly appreciated.

Thanks,
Evan