View a certificate

openssl x509 -in certificate.crt -text -noout

Convert a DER file (.crt .cer .der) to PEM

openssl x509 -inform der -in certificate.cer -out certificate.pem

Verify bundle or chain 

openssl verify -CAfile chainOrBundle.pem ServerCertificate.pem

to test ssl connection

openssl s_client -showcerts -debug -connect <your-ip-address>:<port> -no_ssl2 -bugs