MSMQ - how to identify messages uniquely

Question

Howdy,

I am using MSMQ to asynchronously process messages. The fat client (c# application) would send an object (of a class) to a private queue. The listening service, would listen to the messages and passes it on to a calculation engine which is another .net assembly (c#).

The calculation engine (which is a web service) after processing, will place the results in the form of an object in another queue. A listening service listening for the results will read the message and sends them to the client application.

Question:

How can I uniquely identify a message right from the origination.

thanks

nath