For dynamic IPs you can tracert to the IP. If it's on the old network the last hop before the user's IP will contain the name of a gateway in the format xxx
is ptw, ptn or pcl. E.g. link3-central11.pcl-bng03
.plus.net. On the new network you will see instead one of the points of presence such as colindale.
Unfortunately users with fixed IPs who are on the new network are tunnelled through to the old network gateways so the presence of one of the old gateways doesn't tell you for certain if they are on old or new.
However, for what you are looking at at present, users on fixed IPs on the new network are seeing most of the issues resolved so just look at the ones on the PoPs.
Plusnet user since November 2001
Telephone rental: Pulse8