A dog fight can happen in less than a few seconds and can add up to some serious medical bills if you are not careful and don't know what body language to look for. Dog language is a complex system of physical and verbal cues that let you know if you dog is happy, excited, agitated, fearful, or playful and they express them in the most minor ways that we often miss the cues they are giving other dogs. A quick stare can lead to an all-out, drag down fight as we experienced one night at a dog park. We were walking in, on leash, and passing by a Ridgeback when all of a sudden the dog went crazy and pulled his owner down a hill to attack Foyle. Luckily we were able to get away with no consequences, but I had been watching Foyle's body language over the past few months to see if the hormones were started to kick in and I didn't see him provoke the other dog. I could have missed a quick glance that sent a "challenge" over to the other dog, but from what I saw the dog could have smelled the hormones and that alone could have triggered the attack. The ASPCA has a good article on body language that demonstrates what each body part is expressing during different emotions so you have a better idea of what to look for in your dog.
You can use this to your advantage during your training lessons too because dogs can also read your body cues incredibly well. A good book that describes these cues is Patricia McConnell's The Other End of the Leash. She leads you through what you need to do to remain consistent with your signals through your voice, your body, your motions, and your attitude. Your dogs are constantly looking to you for direction and she guides you through how to think more like a dog and less like a primate. I think I need to read it again it was so good.