Linked servers - Upgrading from 2005 to 2012
More than focusing on other servers that link to your current server, I would worry about applications. In complex environments, some applications may be out of control and keep pointing to your old server.

In these cases, this is the plan I am using:


install the new server with the same instance name as the old one
migrate all server objects, including logins and everything else
switch off or rename the old server
add a CNAME in your DNS to point the old server name to the new
server name


This way, you don't have to worry about linked servers, ETL jobs or applications still pointing to the old server.

Obviously, this assumes that you already have tested everything to be compatible with the new version.

UPDATE

Based on your comments, if you still want to discover incoming linked servers, you can register all your servers and run a multi-server query like this:

select name
from sys.servers
where product = 'sql server'
    and data_source = 'your sql server 2005'