Without rDNS:
nslookup xxx.xxx.xxx.xxx (VPN IP)
** server can't find xxx.xxx.xxx.xxx.in-addr.arpa: NXDOMAIN
With rDNS it might look like this:
nslookup 208.104.203.197
197.203.104.208.in-addr.arpa name = 208-104-203-197.reserved.comporium.net.
ETA: I just pulled 208.104.203.197 out of my firewall. I have beef with them. They hammer me daily from all the way in Gilbert, SC where the fuck that is but I wished they'd stop. I just block their entire CIDR.