I'm not aware of a rule or case book play that covers this. It's been discussed on this board several times. My opinion: If the QB or coach tells me he's taking a knee, our crew will pinch in and remind the defense not do do anything stupid. Once that happens, and the QB starts to take a knee, I'm blowing the whistle and the play is over. I'm not even giving the fake knee play a chance to develop.
If no coach or player tells us they're taking a knee, and they fake it, I don't have a foul.