I use Nirsoft's netrouteview because it has a GUI: http://www.nirsoft.net/utils/network_route_view.html

Private IP Addresses have the following ranges:
10.0.0.0 - 10.255.255.255
172.16.0.0 - 172.31.255.255
192.168.0.0 - 192.168.255.255