Check Certificate File

If you have a certificate file (.crt or .pem) and want to read its information you can do it like so.

openssl x509 -in mycert.crt -text -noout

Checking CSR File

The same thing works for Certificate Signing Requests (CSR)

openssl req -in myreq.csr -text -noout

Check Server-Certificate

If you Certificate is already in use on a server, you can connect to it, to read the certificate-information.

openssl s_client -connect <server>:<port> | openssl x509 -noout -text