Linux
To query for a PTR record of some specific IP, you first reverse the IP address into the canonical form, then query for a record as usual. The canonical form is 194.90.1.5 -> 5.1.90.194.in-addr.arpa . Few examples:
$dig +short PTR  8.8.8.8.in-addr.arpa

google-public-dns-a.google.com. 
$dig +short PTR  5.1.90.194.in-addr.arpa

dns.netvision.net.il.
DIG also supports shorter form of PTR query using -x switch:
dig +short  -x 8.8.8.8

google-public-dns-a.google.com.
Windows
nslookup -type=PTR 8.8.8.8
Server:  dns1-adc.netvision.net.il
Address:  194.90.0.1

Non-authoritative answer:
8.8.8.8.in-addr.arpa    name = google-public-dns-a.google.com

8.8.8.in-addr.arpa      nameserver = ns2.google.com
8.8.8.in-addr.arpa      nameserver = ns4.google.com
8.8.8.in-addr.arpa      nameserver = ns1.google.com
8.8.8.in-addr.arpa      nameserver = ns3.google.com
ns1.google.com  internet address = 216.239.32.10
ns2.google.com  internet address = 216.239.34.10
ns3.google.com  internet address = 216.239.36.10
ns4.google.com  internet address = 216.239.38.10
DNS, DNS-cookbook