After a failed attempt at recalling the name of a childhood acquaintance, Miho helpfully guided Sakura to look at the blackboard where the teacher had the name of the transfer student written down. Sakura continued to feign remembering the guy, but then the scene panned down to reveal that Sakura kept one hand off the table while Miho remarked about Sakura being a liar.
How could Miho tell that Sakura was lying till the end? Was it the timid hand gesture or something she said?
I am not aware of any Japanese body language rooted in having one hand on the table and the other pointed down like that. In fact, Japanese gesture communication is generally conducted at chest-height or higher.
Miho knew that Sakura was lying because she had just finished guiding Sakura to look at the blackboard, because she had been able to tell that Sakura obviously didn't remember his name, and clearly wasn't really paying attention when he was introduced to the class. Miho had just helped provide Sakura with his name since she obviously didn't remember him, so of course she knew that Sakura was lying when she pretended to still remember him.
The camera pan-down was more likely intended to center the view on Miho for her delivery of the 'liar' line than it was to display hand position or body language. And the camera was lowered as far as it was to cut Sakura and the transfer student's heads out of the frame to make it very clear where your focus was supposed to be (on Miho).
If Sakura had actually remembered the transfer student, she wouldn't have needed Miho to point his name out on the blackboard, and thus Miho wouldn't have believed her to be lying when she claimed to remember him.
So, in short, Miho's "liar" line was delivered because she just helped Sakura with the name of someone Sakura clearly didn't remember...then Sakura used that help to pretend she had remembered him all along, pretending Miho hadn't actually helped.
For some people, this might be acceptable because they just helped a friend save face...but it obviously annoyed Miho.
