openssl x509 -in testkey.crt -noout -text

Informations about the used arguments from the OpenSSL man page:

• x509: Certificate display and signing utility.
• -in testkey.crt: This specifies the input filename to read a certificate from or standard input if this option is not specified.
• -noout: this option prevents output of the encoded version of the request.
• -text: prints out the certificate in text form. Full details are output including the public key, signature algorithms, issuer and subject names, serial number any extensions present and any trust settings.

You should redirect the output into a file (with “> testkey_info.txt”) or into some pager (like “| less”) as this outputs a lot of information.