Certificate Transparency

 Certificate Transparency  is an initiative to publicly and verifiably log new TLS certificates to help detect wrong or malicious certificates quickly.

Tools

 * https://crt.sh/ can be used to search for certificates
 * https://sslmate.com/certspotter/ monitors the logs and can send you an e-mail if a new certificate for one of your domains is issued
 * Facebook runs a similar service (Facebook login required)