I will interested to see Ed's response and how it compares to Ahmie's post as it seems that those principle do hold for many dogs; however, based upon my knowledge of my breed - there may be breed differences.
For example - I own Belgians and their natural/relaxed tail state is straight out from the body when in gait. When in a standing position their tails are fully relaxed and straight down. When my dominant/fearful boy is alerting is stands in a perfect square (stacked) and his tail flicks at the end only - if he gets to escalate (i.e. run up to the window and bark at an offender) it is full up and wagging. Ears flat against the head in this breed is submissive - up is normal, straight up and forward is high alert.
So my dog with ears up and tail wagging is NOT a good thing - ears flat and tail straight is for more desirable unless accompanied by some serious growling - in which case my dog is hurt and extremely unpredictable due to the pain factor.
I train for a very short leash heel so the dogs are used to walking in close contact. When he is on alert is vision focuses on that one things instead of constantly scanning the area. He will stare at the object of concern and very quickly flick a glance at me for clues as to how he should be reacting.
For me and my breed, fixation on an object or person is something to be immediately addressed and is often the first clue. Again, I only know my breed this well and am by no means an expert, but there are a lot of clues that my dogs give me long before they get to barking or growling. many of these things I learned from books and training classes (I believe ALL new dog owners should do at LEAST 6 months of dog training - preferably a year) but much of it also came from breed clubs, email lists and breeder forums where new owners could ask questions about their dogs and receive advise and support from people who have worked with these dogs for decades.
The best advise I can give a non-owner is to never, ever assume that a given dog has a good/knowledgeable owner. Or that a dog has been trained. And for heaven's sake - when they tell you "No-you may not pet my dog" please do not assume that there will be an exception made for you and he will suddenly love being petted by strangers becuase you are a "dog person" or becuase "all animals like you."
And for dog -owners... get trained, see Ed (he knows his stuff - we never had a change to train with him but he has an EXCELLENT reputation in the dog world), and BE HONEST. With yourself and with others about your dog.
Sorry - to ramble on - i think i spent too much time in the sun today
