Generally, an issued certificate will have a private key, as one would have been created when you created your certificate signing request (CSR) and submitted it to your certificate authority to generate the end certificate. So it comes down to finding the correct Private Key.
To check if a Certificate has a matching private key, you generally verify that the key corresponding to the certificate is present and correctly associated. This process can vary depending on the system or software you're using. Still, a standard method involves using OpenSSL to compare the certificate's modulus against the private key's modulus. View the FAQ above to see the commands for OpenSSL on how to do this. Otherwise, you can use the above free tool in your browser to verify if the private key you have is the correct one that matches your issued certificate.