Free tool
SSL Certificate Checker
Check the SSL certificate on any website: who issued it, which names it covers, when it expires and how many days are left.
What to watch for
A certificate proves the site is really the one it claims to be and encrypts what travels between the visitor and the server. When it expires the browser stops being polite about it and shows a full page warning, which costs you traffic and trust within minutes.
Check that the names covered include every hostname people actually use, including the www version. A certificate for example.com that does not cover www.example.com will warn on half your visits. A wildcard covers one level of subdomains, so it protects shop.example.com but not deep.shop.example.com.
Most certificates now last ninety days, which makes automatic renewal less of a convenience and more of a requirement. If you are still renewing by hand, that is a reminder waiting to be missed.