In my Outlook 2007 and 2010 addin I was able to determine if the sign button was up or down. using the following code:
signBtn = Item.GetInspector.CommandBars.FindControl(, SIGN, ,True)
if (signBtn.State = Office.MsoButtonState.msoButtonUP) then.
In Outlook 2013 the GetInspector.CommandBars returns nothing. I was thinking that is I could determine in the email is already signed I could accomplish what I needed to do without knowing the state of the button.