How to use the tester
Enter the full address of the page you want to test and click Test URL. Clarnic loads that site's robots.txt, then shows whether the crawler you picked may visit the page and which line made the decision.
The robots.txt is loaded into the editor. Change a rule and the result updates as you type, for every crawler at once, so you can see the effect before you upload the new file. You can also paste a robots.txt that isn't live yet.
How crawlers pick a rule
A crawler first finds the most specific User-agent group that names it, and falls back to User-agent: * if none does. Inside that group, the rule with the longest matching path wins. If an Allow and a Disallow match with the same length, Allow wins. * matches any characters and $ marks the end of the address.
Google Search Console no longer has its own robots.txt tester, so this is a quick way to check a rule.