Chance Meetings
Tara's power wasn't as strong as her mother's, but she was always a reader. Even as a child, her empathic ability was strong. Therefore, when Willow introduced her to Faith-in-Buffy's-Body, it didn't take her long to figure out the ruse.
The girl in Buffy's skin made an impression though. Such an angry, loud slash of aura, screaming from inside. Tara thinks she feels her sometimes, in crowds, on the street. They're destined to meet again somewhere: Faith is seeking her out.