How do you know who or what is locking a remote file?

I can tell what process is holding a lock using Unlocker.

But the problem is that, when there is a lock on a file on a remote machine, Unlocker doesn't show who or what process is locking.

Is there anyway what is holding a lock on a remote file (my remote file is mapped through a network drive)? and also how to remove the lock?

Open up Computer Management, right click (my) Computer and click "Manage":

System Tools > Shared Folders > Open Files

To connect to a remote machine, select "Computer Managment (Local)" in the left pane, then pull up the "Action > Connect to another computer..." dialog, enter the computer name, and hit ok.

You can disconnect sessions from this interface as well.

Process Explorer can search for handle on a file

On a remote machine however, you want to combine PsExec with Handle in order to execute Handle on the distant computer.