In Korea, Kpop idols are pretty aware of which fans are theirs or not, especially if they are a fansite.
Fansite owners tend to try to go to every single event, so the idols see them a lot and if they're only pointing their camera at one member, its pretty clear whose fan they are :)
In this case, a popular fansite of Oh My Girl's Seunghee was at a fansign and for a brief moment turned to snap a video of another member.
Well, Seunghee didn't like that.
Ruh-roh. Caught in the act.
Her reaction is HILARIOUS.
And she doesn't quit either, she keeps it up!
She's clearly being playful (and probably talks to the fansite often at these events haha)