The situation I am picturing is an internet sharing situation where you have machines on a wire to the machine with two NICs. One NIC is for the ether, and the other is for the internet. Using Net::CIDR and to test if it is contained on one of these networks should still work. When you are tracerouting to 10.0.0.1, you could be going out of the Internet NIC, as the lowest eth* will be chosen if not specified, so keep that in consideration.
If the situation is what i think it is, then those machine have one NIC and are on an internal network, that the box in question is directly connected to, so they have no choice but to be on the same network.