Below are the 2 methods which will help you know if the selected document library item is a folder or is the selected list is a Document library or not.
public static class ExtensionMethods
{
public static bool IsFolder(this SPListItem item)
{
return (item.Folder != null);
}
public static bool IsDocumentLibrary(this SPList list)
{
return (list.BaseType == SPBaseType.DocumentLibrary);
}
}