How can you tell if a dog is nervous to the point of being aggressive wag?

A nervous or anxious dog will exhibit the following body language signs to communicate its state:

Flattened ears
Hunched back
Tail between legs
Nose lick

All of these signs show that the dog feels nervous, and will attack if it is pushed further. If the tension is released, you will see the dog take an exaggerated yawn to relieve itself.