Conditional Format - Highlight Cells In Column That Have Entered Numbers Rather Than Formulas

Col A - budget items (description)
Col B - budget dollars
Col C - actual dollars
Row 10 - summation Cols B and C

When the budget is prepared the actual dollars in Col C are equal to the budget dollars for all items. For instance the formula =b2 is placed in cell C2. As time passes the user records actual dollars in Col C for each item by entering the actual amount directly into the cell. Actual dollars do not become known all at the same time, so that Col C will contain a mixture of formulas and entered numbers.

How can I highlight the cells in Col C that have entered numbers rather than formulas.