Easy way to find un-parented stories

You could use work item query to achieve this. There is a filter work items and direct links

For your situation, you need to select Only return items that do not have matching links with parent link types, detail query please refer below screenshot:

Instead of creating a Query you can go to the Backlog and choose "Parents" in the view option menu. In the backlog list at the bottom is a group of unparented stories. This is probably a new feature made after the question was asked.

Adding the image so its easy to find