Using QuickFIX/J I am trying to write a test to time how long it takes for a FIX message to reach its destination. However I am not sure which tag or tags I can use to identify a message as the exact same one that was sent. The toString method for the message seems to return slightly different values.
Are there any tag or tags that are universal to all FIX messages?
There is no unique FIX message identifier like you're looking for.
However, for your perf-test use case, I think you should just pick a String field that belongs to whichever message you are trying to measure, and set it to a unique value that the receiver is looking for.
If you don't have a specific message in mind, I suggest you use the News message (35=B). It doesn't have many required fields and has a very simple structure. You could use the Headline field to set your unique identifier value.