SSL Labs is a free full TLS assessment with terms that restrict commercial use, and 1Lookup returns a certificate verdict in one request for $0.010.

Testing how well a public web server is set up for TLS. The SSL Server Test calls itself a 'free online service' that 'performs a deep analysis of the configuration of any SSL web server on the public Internet', and the API returns the same kind of assessment: a grade, the supported protocols and cipher suites, tests for named attacks such as Heartbleed, POODLE and ROBOT, and revocation and certificate transparency information.

The API licence allows inspecting only sites and servers whose owners have given you permission. Without Qualys's express permission it does not allow commercial use, use on a public web site, or making the API available to anyone other than your authorized employees, and the docs say 'Commercial use is generally not allowed, except with an explicit permission from Qualys'. A hosting provider, certification authority, content delivery network or web development agency can get relaxed terms, on conditions that include offering the scan at no cost to all its customers.

No price is printed: the API page says 'SSL Labs APIs are free to use, with restrictions'. No quota is printed either. The docs say Qualys may limit concurrent assessments and the number of assessments in a period, and answer 429 Too Many Requests when new assessments come too fast. Cached reports do not count against the limit on new assessments.

2 credits per certificate read, free when the domain has no certificate to read. On the Starter plan ($99 a month for 20,000 credits) a check is about $0.010, which covers 10,000 certificates a month and is $9.90 per 1,000. At the Enterprise end of the rate card it is about $0.004. SSL Labs is the cheaper bill: it is free to use under its terms, against $0.010 for every certificate 1Lookup reads.

The SSL Labs docs say server assessments 'usually take at least 60 seconds', and ask clients to poll every 5 seconds until the assessment starts and every 10 seconds until it finishes. 1Lookup answers in one request and one response, from one connection with an 8 second clock for the whole attempt.

A grade (A+ down to F, with T for no trust and M for a certificate name mismatch), the supported protocols and cipher suites, tests for named attacks such as Heartbleed, POODLE and ROBOT, a revocation status with CRL and OCSP addresses, whether OCSP stapling is deployed and whether certificate transparency information is present. 1Lookup reads the certificate and the connection it came over. It does not test or grade the server, check revocation or search transparency logs.

SSL Labs returns certificate problems as a bit mask (issues such as no chain of trust, hostname mismatch, revoked and self-signed) and dates as Unix timestamps, so a caller turns them into a plain verdict itself. 1Lookup returns the status in words, the days left as a number, dates as YYYY-MM-DD and each problem as a sentence.

API v4 needs a registration call first, with a first name, last name, organization and an organization email, and the docs say 'We do not allow the use of email services such as Gmail, Yahoo, or Hotmail'. Calls then send that email in a header, and an unregistered call returns 441. 1Lookup uses one API key for every product.

On depth and price. It is free to use under its terms, it assesses the protocols, cipher suites and named attacks that 1Lookup does not test, it reports revocation status and certificate transparency information, it has an open-source client (ssllabs-scan), and its API page says it allows 'scheduled and bulk assessment'. 1Lookup reads the certificate only, and charges for every certificate it reads.

If you need the full TLS assessment and a grade for a server you are allowed to test, SSL Labs is the right tool and it is free. If you want to check certificates inside your own product or at volume, in one request with a plain verdict and the days left, bulk CSV files and one balance that also pays for phone, email and IP checks, 1Lookup's SSL Certificate Check bills 2 credits only when a certificate is read.

For testing a server you own or have permission to scan, yes: the web test is free and goes deep. For a service that reads certificates at volume or inside a product, read its terms against your use first, then compare a price per certificate read.