Simply run this command, replace example.crt with your actual certificate file:
$ openssl x509 -noout -text -in example.crt
Certificate:
Data:
Version: 3 (0x2)
Serial Number:
..
Signature Algorithm: sha256WithRSAEncryption
Issuer: C=GB, ST=Greater Manchester, L=Salford, O=COMODO CA Limited, CN=COMODO RSA Domain Validation Secure Server CA
Validity
Not Before: Sep 10 00:00:00 2014 GMT
Not After : Sep 10 23:59:59 2015 GMT