A sniffer like Ethereal or any other syslog tools of the own device (Servers
and Routers) could grant a good reply report regarding the old address. If
there are no reply to the old address, you may can turn off the link.
OK. I mentioned traffic capture systems; I think you're additionally saying
"ping scan the network and see what replies".
That certainly would verify that the old prefix has been removed. It seems
to me, though, that this really isn't the question. The question is whether
there are any sessions remaining addressing applications via the old prefix
- if someone is still using it, you don't want to remove it yet, and after
they have all stopped, it doesn't really matter whether it is there or not.
Hence, it seems to me that once the last session using an address goes
quiescent, the address can be removed from the system. When there is no
longer any traffic using the prefix, you can remove the prefix from the
router serving the prefix.