The following example uses the hWnd property with the Windows API IsZoomed function to determine if a window is maximized.
' Enter on single line in Declarations section of Module window.
Declare Function IsZoomed Lib "user32" (ByVal hWnd As Long) As Long
Sub Form_Activate()
Dim intWindowHandle As Long
intWindowHandle = Screen.ActiveForm.hWnd
If Not IsZoomed(intWindowHandle) Then
DoCmd.Maximize
End If
End Sub