Issue 'On Error Resume Next' and try to retrieve the Combo object using the iGrid.Combos("...") call. If it doesn't exist, an error occurs.
Here is a handy function implementing this idea:
Private Function ComboExists(ByVal psComboKey As String) As Boolean
Dim oCombo As ComboObject
On Error Resume Next
Set oCombo = iGrid1.Combos(psComboKey)
ComboExists = (Err.Number = 0)
End Function