How to Verify an SSL Certificate

SSL (Secure Sockets Layer) certificate is a security protocol used to ensure secure communication between websites and their users. When an SSL certificate is properly installed on a website, data is transmitted encrypted, which builds trust for e-commerce sites, blogs, and corporate pages. However, you should regularly verify that an SSL certificate is functioning correctly. This guide explains, step by step, how to verify an SSL certificate and what checks to perform to keep your site secure.

Quick SSL Check in the Browser

The fastest and simplest way to confirm whether a website’s SSL certificate is valid is to use your web browser.

To check in a browser:

Verify the site URL starts with https://.
Look for a padlock icon at the left of the address bar. Click it to view certificate validity and the identity it was issued to.
Check the certificate’s validity dates and the Certificate Authority (CA) that issued it.

If the padlock is replaced by a warning icon, the SSL certificate likely has a problem: it may be expired, misconfigured, or otherwise invalid.

Verify with Online SSL Checkers

For a more detailed inspection, use free online SSL testing tools. These services scan your domain and return reports about certificate validity, chain configuration, supported protocols, and common vulnerabilities.

Typical output from these tools will show whether your certificate is installed correctly, whether intermediate certificates are present, and any issues to address.

Command Line (Terminal or CMD) SSL Checks

Server administrators and developers can inspect SSL details directly from the command line. This gives raw output about the connection and presented certificate.

Linux and macOS terminal command:

openssl s_client -connect yourwebsite.com:443

Windows (CMD or PowerShell) command is the same if OpenSSL is available:

openssl s_client -connect yourwebsite.com:443

This command opens an SSL/TLS connection and prints certificate details, including issuer and validity dates.

Check the SSL Certificate Chain

Certificates form a chain that leads back to trusted root authorities. If any link in that chain is missing or incorrect, visitors will see certificate errors.

A complete chain consists of:

Root Certificate: The top-level trusted authority.
Intermediate Certificates: Link root authorities to end-entity certificates.
End-Entity Certificate: The certificate issued to your website.

You can check the chain in the browser certificate viewer or using online tools to ensure no intermediate certificates are missing or misordered.

Validate Certificate Expiration and Dates

Many SSL issues occur because a certificate has expired. To avoid service interruptions, always check the certificate’s start and end dates.

Click the padlock in the browser and view certificate details to see issuance and expiration dates.

From the command line you can inspect a certificate file with:

openssl x509 -in certificate.crt -noout -dates

An expired certificate will trigger “Not Secure” warnings and can significantly reduce site traffic and conversions.

Confirm the Issuing Certificate Authority

Certificates are issued by Certificate Authorities (CAs). Trusted CAs are recognized automatically by browsers. Verify the issuer field to ensure your certificate comes from a recognized CA.

In browser certificate details, find the “Issuer” entry and confirm it’s a known CA.

Certificates from unknown or untrusted authorities will be flagged by browsers and produce errors for visitors.

Ensure the Certificate Matches the Domain

SSL certificates are valid for specified domain names. If the certificate was issued for a different domain, browsers report a “domain mismatch” error. Pay special attention to www vs non-www variations and any subdomains.

Check the certificate’s Common Name (CN) and Subject Alternative Names (SAN) fields to confirm the domain you’re accessing is included.

Wildcard and Multi-Domain Certificates

If you use a wildcard certificate (for example, *.example.com), ensure it covers all intended subdomains such as sub1.example.com and sub2.example.com. For multi-domain (SAN) certificates, verify that each listed domain is correct and in use.

Test Browser and Device Compatibility

Some older browsers and devices do not support modern TLS versions or certain certificate features. Use compatibility tests to confirm users on older platforms won’t encounter problems.

Online compatibility checks and manual tests on multiple browsers and devices help identify platform-specific issues.

Post-Installation Checks

After installing a certificate, run the following checks:

Is HTTP correctly redirected to HTTPS?
Are there any mixed-content warnings on pages (HTTP resources loaded on HTTPS pages)?
Are other services (mail servers, APIs) using SSL/TLS correctly?

Performing these checks ensures all aspects of your site and related services are protected with secure connections.

Verifying an SSL certificate is essential to protect your website and your users’ data. Start with a simple browser check, then use online tools and command-line methods for deeper inspection. The most important factors are a valid expiration date, correct domain coverage, and a complete certificate chain issued by a trusted CA.

When your SSL certificate is valid and correctly configured, you gain user trust, improve search engine standing, and increase conversions on transactional sites. If you discover an issue, contact your certificate provider or hosting support to correct the configuration promptly.