You can use nslookup
on the IP. Reverse DNS is defined with the .in-addr.arpa
domain.
Example:
nslookup somedomain.com
yields 123.21.2.3
, and then you do:
nslookup 123.21.2.3
this will ask 3.2.21.123.in-addr.arpa
and yield the domain name (if there is one defined for reverse DNS).