Conditional Formatting for Cells Containing Formulas

I am working on a project milestone sheet and when projects are first entered the milestone dates are calculated using formulas, based on the start date. As the project gets closer, we will have users manually overriding the formula with the 'actual' date if it is changing. For this solution, we are not using column formulas, but just have them in the projects that are scheduled farther out.

We want to be able to see whether a cell currently contains a formula or if it's just a date that someone has entered. Does anyone have an idea on how that could be conditionally formatted?

I attempted to use the paramater contains an equals (=) sign, but that didn't work. Curious to hear others thoughts.