Thanks for your question. This is a very interesting one.
There are a few things that we should discuss before we try to classify normal vs abnormal.
First, there are many itchy pets out there. I often believe that pets lick because it is their way of scratching. So I always ask about and examine for any other signs of allergies or itchiness? These include face rubbing, hair loss in any other areas, examining for fleas, examining for red skin, especially the ears, feet, face, and I even check anal glands and the rectum.
Second, we talk about any other odd behaviors that you might be noticing. I ask of there is a pattern to the licking? (I had a client just the other day tell me that their Corgi licks his front left foot every night after dinner. Nothing else just that foot and just at that time of day). I also try to discuss the pets environment, diet, supplements, etc. It is a long discussion of question and answer to try to correlate behavior with disease.
There are also some forms of neurologic disorders that can be manifested as odd behaviors that could look like air licking. A veterinary neurologist can be an invaluable resource for these types of cases. We call these odd behaviors with a neurologic origin as "complex partial seizures".
I also ask about whether you have the ability to videotape the episodes? Often a picture speaks a thousand words and the video that you get at home is often not able to be reproduced in the limited and foreign environment of the vet hospital.
I just read an article about the different kinds of seizures we see in dogs, by Dr. Nicholas Dodman, titled "Complex Partial Seizures Or Compulsive Behavior." The link to the article is below.
I think that you are trying to specifically identify "normal" for your dog, but that isn't always so easy to categorize. Normal can be a silly or odd behavior, a dermatologic condition, an internal organ disease that manifests as an odd behavior, or a neurologic condition. The only way to identify the differences is with the help of a veterinary professional. That should start with a thorough examination, an in-depth discussion, and perhaps some diagnostics.
To help you decide whether you need to visit the vet I would say, "if your dog is licking and not causing any self trauma, or if the behavior is described as "extreme, irrational, or apparently unprovoked" then it may be normal for your dog." I do, however, always feel that it is best to play it safe and make an appointment to see your vet and ask these concerns.
If you would like to discuss any of these further I can be reached at the clinic.
Thanks for asking, and best of luck to you and your pug.
Krista Magnifico, DVM
Owner Jarrettsville Veterinary Center