I think there's a certain amount of animal instinct required to assess these kinds of situations.
A couple of years back I was attacked by a dog in the park behind where I used to live. We just knew that it was being agressive as it approached us, even though there wasn't any outward signs. Some survival instinct picked up things that I'm still not aware of.
Fast forward to a couple of weeks ago and my wife and I were walking in the woods when some {bleep} failed to keep his dog (probably only about 9 months old) under control. It charged at us and then jumped up, but I knew instinctively that it was only being excited and playful and that the worst I could expect was a bit of slober and some mud on my t-shirt.
Outwardly, the two dogs probably appeared to be behaving exactly the same.
