You don't need to understand a lot about VBA to use a pre-written user defined function (UDF).
Here is a step-by-step guide for Excel 2007:
As it sounds like you are distributing the workbook, you will need the UDF to stay with the workbook.
Save the workbook in a macro-enabled format. *.xlsm
Right-click the name tab at the bottom of the worksheet and select 'View Code'
This opens the Visual Basic window, with your worksheet name selected in the Project Explorer pane.
Right-click the worksheet name and select 'Insert' and 'Module' (not Class Module).
In the large visual basic window (the one with two drop-downs at the top), paste in this code.
Public Function ISFORMULA(TestCell As Range)
If TestCell.HasFormula = True Then
ISFORMULA = True
ISFORMULA = False
Click Save from the VB menu bar
Now use Alt + f11 (the Alt key and function key #11 together) to return to the main Excel window.
In an empty cell enter a formula like this:
The cell will return True or False as described in my previous post.