How do I determine the spid of the session that is making a query on a linked server?

I have a session running on a SQL Server instance that is blocking other sessions (holding on to a lock without releasing it).

Querying sys.sysprocesses yielded the hostname of a different SQL Server instance.

I want to know that spid of the originating session that is ultimately responsible for blocking the sessions on the linked server.

• How would I do that?