Highlight Cells with Formulas - Works on entire active worksheet
Sub Highlight_Formulas()
'Highlights all cells with formulas on the active sheet
'Will remove color from cells without formulas
Dim r As Range
With ActiveSheet.UsedRange
    .Interior.ColorIndex = xlNone
    For Each r In .Cells
        If r.HasFormula Then r.Interior.ColorIndex = 6  ‘yellow
    Next
  End With
End Sub
Highlight Cells with Formulas - Works on Predefined Range of Cells
Sub Highlight_ Formulas_Range()
'Highlights all cells with formulas within a selected range
'Will remove color from cells without formulas
Dim Rng As Range
For Each Rng In Range("A1:B25")  'Range to highlight cells
     If Rng.HasFormula Then
     Rng.Interior.ColorIndex = 3 'red
   Else
     Rng.Interior.ColorIndex = 0 'blank
   End If
Next Rng
End Sub