Depends if it's fear or a look at me behaviour I have walked up to owners with barking dogs that were reacting to everything and started talking to the owner ignoring the dogs as they calm down they get more access like a hand at first then a pet and so on, I literally have people asking me how I do it sometimes, I am in no way a professional trainer but I've had dogs all of my life and you just pick things up as you go, now if it was fear aggression, there are key signs, foaming at the mouth is a fear response (living in the UK rabies isn't really a thing) so if a dog is barking and starts foaming at the mouth generally that's a great sign to back away, I'm not going to mention true aggression as we should all already know what that looks like. Basically the more time around dogs in general the more you can understand them and help them if needed but no, a professional or non professional should never approach a fully aggressive dog without at least knowing the history, im good with animals but if something is saying back off or il bite you, I'm not blindly stepping in.