Thanks James, the extension we used came from the Azure marketplace, it's called Split!: https://marketplace.visualstudio.com/items?itemName=blueprint.vsts-extension-split-work.
The documentation is fairly minimal but I did create a query that looks for the word "split" using the "contains words" as my operator which is common in all stories that are split, I could have also created a Tag called "Split" so looks like I'm good to go here. Thanks for your help.