DNS

We just did something like this. Given a list of your servers in CSV format, you can do this to find who has what DNS servers set:

t = Import-Csv C:\TEMP\PS\Servers.csv
$t|
%{
$computer = $_.Name.trim()

$DNS = WmiObject win32_networkadapterconfiguration -computername $computer -ErrorAction SilentlyContinue |?{ ($_.IPAddress -like '192.168.*')}
$x = $DNS.DNSServerSearchOrder
write-host $computer " : "$x

}

Then, if we were looking to make sure that 192.168.1.1 and 192.168.1.2 were in the DNS settings, we had:

$t = Import-Csv C:\TEMP\PS\Servers.csv
$t|
%{
$computer = $_.Name.trim()
$lan = Get-WmiObject win32_networkadapterconfiguration -computername $computer |?{($_.DNSServerSearchOrder -notcontains '192.168.1.2') -and ($_.IPAddress -like '192.168.*')}
$dns=@("192.168.1.1","192.168.1.2")
$lan.SetDNSServerSearchOrder($dns)

}

You can modify this to your own preferences.

You can set the DNS-servers to log which clients are querying it, or you can setup a network sniffer on the DNS-Server to see the same.

Just only kill one DNS-server at the time.
There still is the risk that some client is using one of the decommisioned servers as secondary DNS, but you will only know if that client's primary DNS dies.

If you run wmvare or other virtualization, you can make a forwarding DNS-server with all the ip's of the decommisioned servers, and in that log discover who still uses the old servers. Heck, a forwarding dns does not use much cpu-power, and you could have it around for long time until you are sure noone is using it anymore.

Dunno about wins though...

Of cause, if it is on a private network, you can just give the existing DNS servers the IP's of the decommisioned servers as secondary ip's, but that is a dirty solution.

why not just set your routers to redirect traffic from the old servers to the new ones via entries in the routing table?

In a worst case scenario of not being able to find what accesses what, just slip an extra NIC into the servers you're keeping and give that NIC the IP address of the old server you're taking out.