Nick Heppleston has a post on how to identify the BizTalk host by correlating the process identifier (PID) to the results of executing tasklist. Check out his post for the details.
From an operational perspective, its pretty easy to fire up Process Explorer in place of Task Manager to get a wealth of information about the processes running on your system. Check out the Windows SysInternals Suite site for more information and downloads.
Each BizTalk host is started with a set of command line parameters passed to it. One of these parameters will be the name of the BizTalk Host that the process is an instance of. You can view these command line parameters via Process Explorer and identify which hosts are consuming the most CPU, Memory, etc.
Once your running Process Explorer, add the command line column (right click column-header -> check command line option), and that view will become saved as part of your user preferences.
Here is a snapshot:
Notice that you can see the two BizTalk host instances 'Tracking' and 'MessagingReceive' that are running on this server.