Navigating the openssl suite of subcommands is time consuming.
Here's my list of frequently used commands for easy reference:
Examining the certificates being offered by a host (replace imap/https with any service, -starttls imap can be eliminated if you are checking a service that provides tls from the start):
openssl s_client -starttls imap -showcerts -connect chavez.mayfirst.org:imap openssl s_client -showcerts -connect chavez.mayfirst.org:https
Examine a certificate signing request:
openssl req -text -verify -noout -in /path/to/file.csr
Examine a x509 certificate:
openssl x509 -noout -text -purpose -in path/to/file.crt