There are few things that have to be kept in mind here.
First, Faith might not have been highly educated, but that isn't the same thing as being stupid, which is exactly what she would have been if she were actually trying to pass herself off as Buffy, and, yes, she knew Buffy well enough from their time together to have been convincing in the role if she had wanted to, at least for as long as it would have taken for her to leave town.
Basically, Faith was trying to ruin Buffy's character, to make her friends so angry and disgusted with her that they wouldn't try again, as they had done just a couple of years earlier, to locate her and bring her home. As far as she knew at the time, she was going o be occupying that body permanently from that point on, so he last hing she would have wanted would have been to have Joyce or the Scoobies trying to track her down. Faith wanted all connections between her new body and its former occupant's friends permanently severed. Faith was always blunt, but never actually rude, so her acing that way in the Bronze was exactly that. An act, the same as her slut performance was with Riley, and, to a somewhat lesser degree, with Spike. Remember,Tara didn't know Buffy, and Faith was aware of the fact, because she asked. But she DID know, just by observation, that Tara was special to Willow, so hurting her was another way of destroying Buffy and Willow's friendship. What she DIDN'T know was that Tara could sense the fragmenting of Buffy's aura. If she had, she would never have spent more time with her than it would have taken to say, "Vamp alert. Gotta split." and headed for the door as quickly as her legs would carry her.
As for Tara being able to read Faith/Buffy's aura, I seriously doubt that she used that power on a constant basis any more than I believe that Superman uses his x-ray vision constantly. It was simply there when she needed it. At best, she saw the way that Faith/Buffy treated Willow, who had assured her was her closest friend, sensed something was wrong, and decided to take a closer look. The irony here was that, if Faith hadn't been trying to destroy willow and Buffy's friendship by being intentionally rude to someone she recognized as being very close to Willow, had simply treated her civilly, she could have pulled the whole thing off without a hitch.