You can check to see if you have migrated to Shopify's Checkout Extensibility post-purchase pages by visiting Shopify Admin > Settings > Checkout.
There will be a warning at the top of this page if you HAVE NOT yet migrated your store's post-purchase pages to Checkout Extensibility.
Here is how the top of that page will appear if your store HAS migrated your post-purchase pages to Checkout Extensibility:
To learn more about how easy Google Customer Reviews works with Checkout Extensibility, take a look at our article that explains how the survey opt-in pop-up works with Checkout Extensibility.