Teething fever or flu?
Basically, I would look for other corresponding symptoms. If the baby has a runny nose, coughing, or vomiting, I would tend to think of the fever as being illness-related, and treat accordingly.
However, if the fever is on its own, or I have seen my daughter chewing on her hands lately or have an outbreak of diaper rash, I would classify it as related to teething.
I think teething fevers are usually pretty low-grade, whereas fevers caused by illness can be low or high. If I'm wrong, please do correct me in the comments!
And finally, in all honestly, I treat both with ibuprofen (unless there is vomiting) with a dosage appropriate to my baby's weight.
I rarely treat symptoms with ibuprofen or Tylenol, I'm nervous about using anything over the counter in my babies or children...unless the fever was extremely high. I was glad to find essential oils as I can treat all those symptoms without fear of harmful side affects.