Terminal.app経由で次のように入力します:
openssl x509 -noout -text -in ~/Desktop/yourcertificate.crt  
最後のパラメーターは証明書ファイルのパスです(必要に応じてドラッグアンドドロップできます)
私たちに与えます: 
Certificate:
    Data:
        Version: 3 (0x2)
        Serial Number: 27 (0x1b)
        Signature Algorithm: sha1WithRSAEncryption
        Issuer: C=US, O=Apple Inc., OU=Apple Certification Authority, CN=Apple Root CA
        Validity
            Not Before: May 26 19:16:09 2010 GMT
            Not After : Jul 26 19:16:09 2017 GMT
        Subject: C=US, O=Apple Inc., OU=Apple Certification Authority, CN=Apple Application Integration Certification Authority
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
            RSA Public Key: (2048 bit)
......