Sub CheckFormula()
Dim iCell As Range
    For Each iCell In Selection.Cells
        If Not iCell.HasFormula Then iCell.Interior.ColorIndex =  6 
    Next
End Sub

Private Sub Worksheet_Change(ByVal Target As Range)
    If Not Intersect(Target, Range("A:A")) Is Nothing Then
        If Not Target.HasFormula Then Target.Interior.ColorIndex =  6 
    End If
End Sub

Private Sub Worksheet_Change(ByVal Target As Range)
    If Not Intersect(Target, Range("A:A")) Is Nothing Then
        If Selection.Cells.Count >  1  Then Exit Sub
        If IsEmpty(Target) Then
            Target.Interior.ColorIndex = xlNone
            Exit Sub
        End If
        If Not Target.HasFormula Then
            Target.Interior.ColorIndex =  6 
        Else
            Target.Interior.ColorIndex = xlNone
        End If
    End If
End Sub

Private Sub Worksheet_Change(ByVal Target As Range)
    If Not Intersect(Target, Range("A5:A8")) Is Nothing Then
        On Error Resume Next
        If Not Target.HasFormula Then
               Target.Offset(,  2 ).Font.ColorIndex =  3 
        Else
            Target.Offset(,  2 ).Font.ColorIndex =  0 
        End If
    End If
End Sub