If I remember correctly, it's the position of the tail, and not whether it is wagging or not. I took a class with a certified behaviorist when I worked for the humane society since we did temperment testing and were the first people to come in contact with these dogs. If the tail is straight up, that's a warning, the tail should be at a more relaxed position, but not tucked (that could end up being a dog with fear based agression).
I have the same problem with both of my goldens and we discussed this on another thread a while back. I walk my boys on gentle leaders and I don't encourage them to interact with other dogs anymore since they are really isolated where we live and are rarely around other dogs, so my goal is to get them to ignore other dogs we come in contact with. When we are walking and we come up to another dog, I tell my boys to "leave it", a command they already know and as long as they keep walking past and focus on me and what I am saying, I keep telling them they are good boys and reward them. If they start reacting to the other dog, I just keep walking and don't allow them to stop, and as soon as they get their focus back on me, I reward them and we still keep going.
Training classes are a great way to get through this because not only will she learn her manners (or perfect what she already knows) but she will constantly be exposed to other dogs if you go to a group training (agreed against pet smart type classes, I don't know what type of qualifications the trainers there have) but that's another good place to expose your dog to other pets, just walk through a pet store, but be sure you have control before trying that
I highly recommend the gentle leader, my dogs are so much more attentive to me and better behaved when they have them on. But beware, your pooch will likely not enjoy the feeling at first, so you just gotta put it on and go, and if she stops and has a temper trantrum (and it will happen) you just have to keep walking so she has to follow or be pulled along...she will stop the fit really quickly when she finds out it won't get you to stop or take it off or even pay any attention...don't react in any way until she stops misbehaving and starts walking with you and then tell her she's a good girl.
Sorry so long, I hope this helps.
Good luck.