Stick 'em up
If your class is likely to be observed, take the following precautions.
Tell the students that when the inspector is in the room you will ask them some questions. Every time you do, they are all to stick their hands in the air.
If they know the answer, they are to raise their right hand. If not, they must raise their left. You can work the rest out for yourself.
The only thing is, though, apparently the "raise your hand if you know the answer" method of teaching is isn't considered to be, er, good practice.
But, presumably, when presented with a class full of students who know the answer to every question, even an Ofsted inspector might turn a blind eye to a few old-fashioned teaching methods.