If you add a widget via Add a Gadget link in Layout, Blogger will automatically generate an Id for the widget without telling you what the Id is. To find the Id, you'd normally have to search inside the template HTML code.
Well we have found a simpler and more straightforward way without going to your template HTML. As an example we are going to check the widget Id and the corresponding section Id. Follow the steps:
1- Log in to your Blogger Dashboard account
2- Go to Layout and locate the widget
3- Click the Edit link and widget configuration window will pop up.
4- Notice there is an URL of the widget on top of the window. What we are looking for is somewhere within that URL
5- Now we can see the URL in full (Click for larger picture)
This is the URL:
https://www.blogger.com/rearrange?blogID=5900390404651267310&action=editWidget§ionId=sidebarrightnbt&widgetType=null&widgetId=HTML4Here they are, sectionId=sidebarrightnbs and widgetId=HTML4. So the section Id is sidebarrightnbt and my widget Id is HTML4.