Determine if a certificate is valid using Keychain Access on Mac
In the Keychain Access app on your Mac, click Certificates in the Category list, then double-click the certificate you want to evaluate.
Choose Keychain Access > Certificate Assistant > Evaluate [certificate name].
Select a trust policy:
To evaluate an email certificate, select S/MIME, then type the email address of the sender.
To evaluate a web server, select SSL, then type the host server’s URL. To get the certificates, select Ask Host For Certificates.
To evaluate other types of certificates, select Generic.
To view the information about a signed certificate, select Code Signing.
For an explanation about certificates and trust policies, click Learn More.