Just a suggestion
When testing a campaign, the subject line of the email is appended with the word " - Preview". Something like that.
So for example my subject "Exhibition Invitation" would appear as "Exhibition Invitation - Preview" when sending a test.
However I think the word "Preview" is a bit ambiguous. I've had some clients go off the deep-end thinking I've posted without their final approval. Maybe it would be better to use the word "TEST" or "TEST ONLY". It might be easier to spot.