Reverse-DNS/PTR is used by mail servers. It is used to map an IP address to FQDN or hostname.
To find Reverse DNS record for an IP address, run
nslookup IP_ADDR_HERE
In this example IP 51.38.246.115 have a Reverse DNS record ok.serverok.in
Using dig
You can also use dig command to find PTR record for an IP address.
dig -x IP_ADDR_HERE dig +short -x IP_ADDR_HERE
Using host
host IP_ADDR_HERE
See DNS