query about determining ingress interface

Hi Is there a way for an ISP to determine the ingress router interface at its network border that will carry IP traffic _from_ an IP address not owned by it? I don't want to assume the path is the same in both directions, and tools such as CAIDA's skitter plot paths from specific sources. One approach might be deriving network paths from CAIDA's data, perhaps someone has already done this? Thanks. Rajesh.