YDone - Navigate to Special:Version on your wiki to verify that the extension is successfully installed.
To verify that it's working, try creating an account for username "Adm1n"; most wikis have an "Admin" account and so AntiSpoof should reject this lookalike username with a message like "The name Adm1n is too similar to the existing account: Admin".
This extension is being used on one or more Wikimedia projects. This probably means that the extension is stable and works well enough to be used by such high-traffic websites. Look for this extension's name in Wikimedia's CommonSettings.php and InitialiseSettings.php configuration files to see where it's installed. A full list of the extensions installed on a particular wiki can be seen on the wiki's Special:Version page.