Reverse traceroute sites.
Do you know of any sites that will do a traceroute back to me, as opposed to me initiating the traceroute from my location?
One of the best sites is at traceroute.org. Google is also your friend here - try using the search terms: traceroute gateways
Many sites here allow you to trace from a remote destination back to your own computer (or another server) to help determine the return route.