Outlook Spy is a developer tool for MS Outlook that you can install as an add-in. Download it here: http://dimastr.com/outspy/home.htm .
If you select a message, and then click on iMessage, and then do the same for another message you'll be to compare them and detect any differences.
If you click on the Compare tab for one message, and then drag over the PR_ENTRYID field from the other message on to this tab . . .
. . . .click on Show Properties . . . different on this object, and the fields which are different on first message will be displayed.