I've had the same need and you just can't get the dom to reliably differentiate. Here's what I did, works well....wouldn't advise if you've got lots of them as performance would be a problem:
this looks at the text that sits between your start and end select tags and if - somewhere in there - you have an item that is selected. From there you can work with the string "<" to figure out your index. (i only needed to know if SOMETHING was selected so I didn't have to do much parsing).
Hope that helps. It ain't pretty...but...it works. It'll return -1 if there is no "selected" item in your html.