Andy Morrison wrote about a technique for identifying the correct host instance to attach to when debugging components associated with orchestrations.

A potentially easier solution is to keep perfmon running with the counter shown below (BizTalk:Messaging/ID Process - all instances.)  The counter values will update every time a host instance recycles.  The Debug-Processes dialog in Visual Studio will show you (and allow you to sort by) process IDs to make this easy. 

Then put PerfMon in "report view" to easily see process IDs by host instance: