You can use this to check if the reverse DNS record has been registered.
Note that sometimes rDNS providers won’t accept a record which isn’t linked to a working DNS one. That means you should:
own the domain
register a regular DNS record (aka A/AAAA record) to the relay’s IP (use this to check)
lastly, add the rDNS record (aka PTR record) to the hostname you decided to use