You can check by visiting the site in your browser and viewing the certificate that the browser received. The details of how to do that can vary from browser to browser, but generally if you click or right-click on the lock icon, there should be an option to view the certificate details.
In the list of certificate fields, look for one called "Certificate Signature Algorithm". (For StackOverflow's certificate, its value is "PKCS #1 SHA-1 With RSA Encryption".)