Reverse DNS Check
You can use this reverse DNS tool to find and validate the matching PTR record(s) of any IP address.
What is Reverse DNS?
In a nutshell, reverse DNS translates an IP address (ie, 126.96.36.199) into a hostname (ie, one.one.one.one). This is done through the use of PTR type DNS records, and is essentially the opposite of regular 'forward' DNS queries which translate a given domain name into a computer-friendly IP address.
An IP address is said to be forward-confirmed if the A record it's PTR record resolves to matches the original IP address.
RFC 1912 (Section 2.1) recommends that all internet-reachable hosts should have a "name". In practice, there are plenty of servers on the internet with no/misconfigured reverse DNS settings. One would most likely worry about reverse DNS configuration when setting up and managing a mail server, as they are often referenced when checking whether email is spammy, or coming from a reliable source.