If you simply call NET SHARE (no parameters) from a DOS prompt you get a list of all shared drives. Some of the items in the list are rather confusing to me (I am using XPP), but if I add a shared folder with explorer and then use NET SHARE at the DOS prompt, I see my share in the list. If you wanted to check if a folder was shared, you could check for its name in each line of the NET SHARE output with MatchStr.
Can you elaborate on that? If I call net share on an already shared folder, it will still allow the folder to be shared again. It only throws back an error if you try and share it or any other folder using the same share name.
If I use 'Windows Explorer' to manually add (with the mouse... no automation) a shared drive (via Tools > Map Network Drive), then I see this drive mapping from the DOS prompt afterwards when I run NET SHARE (with no parameters).