Paste a URL above. If one block declares BreadcrumbList and parses, your breadcrumb markup is on the page and the JSON is readable. The check runs on the page as it is served, so you see what Google gets. It doesn't check the properties inside the block. Use Google's Rich Results Test for that.
Crawling, sitemaps and structured data
Breadcrumb schema validator: check BreadcrumbList is on the page and parses
Enter the URL of a page with breadcrumbs. Look for BreadcrumbList in the types each block declares, and check that the block parses.