This is a quote from Harry Potter and the Half-Blood Prince:
There was a pause and then Snape said quietly, "Ah . . . Aunt Bellatrix has been teaching you Occlumency, I see. What thoughts are you trying to conceal from your master, Draco?"
So it seems that Prof. Snape could tell Draco had something to hide. Is it possible to tell if Occlumency is being used to hide the truth?