There is a lot to be learned by paying attention to a pug's tail. I believe it is a true indicator of how your pug is feeling. For instance, a tightly curled tail indicates a happy and healthy pug. The tighter the curl the happier the pug, I have found.
A pug tail that is at half-mast slightly droopey or unwound, may be a sign of indifference to a situation or possibly the puggie isn't feeling too well. Alas, if you ever find your pug unable to hold up his tail at all and it remains down for any length of time during his waking hours, you can be sure that he is either depressed, scared, or ill.

I have had Pugsley for 10 years now and have come to learn a lot about how he is feeling, his health, and well-being simply by observing his tail.

To demonstrate, I have taken some pics of Pugsleys tail at different times:

I find this to be yet another wonderful attribute of the Pug dog's many charms. Obviously a dog cannot talk to you and tell you how is he feels. Oftentimes, problems go unnoticed as a result. With a Pug however, you need only look to the curly pretzel as a tell-tail sign ;-)