You can use an OpenSSL s_client -connect command to check a certificate on a remote server by specifying the remote system in the form
x.x.x.x:portwhere x.x.x.x is the IP address of the remote system and port is the relevant port or you can use the fully qualified domain name (FQDN) in place of the IP address. E.g., I used the command below to check the status of a certificate I obtained from Let's Encrypt, a "certificate authority that launched on April 12, 2016 that provides free X.509 certificates for Transport Layer Security (TLS) encryption via an automated process designed to eliminate the current complex process of manual creation, validation, signing, installation, and renewal of certificates for secure websites." The server I checked functions as a POP3S server using port 995, so that was the port I specified.
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