I guess I'm a little late to the party, but the reason Toph did not call out Katara's lie was because she did not check. Katara was not above Toph's threshold of lie detection as evidenced in Lake Laogai - "He was not my boyfriend" "I can tell you're lying". And even then, Katara was not technically lying since she and Jet never really dated, but she did have a pretty big crush on him. That being said, Toph's Seismic Sense was good enough to detect lies that were even technically not lies, just deliberate misleadings of information, unless the subject in question thought they were telling the truth - "They both think they're telling the truth" (Lake Laogai), or if they were extraordinarily good liars - "Okay, you are a pretty good liar" (The Day of the Invasion, Part 2). However, in order to detect lies, a user of Seismic Sense (such as Toph) must consciously search out lies. Therefore, they need to have reason to believe the subject is lying, and Toph did not, and she was also tired as she had just woken up, so she didn't check.