When the meringue is thick and shiny and holds firm peaks, turn off the beaters and very slowly invert the bowl. If the meringue slides or shifts, even a bit, it’s not yet stiff enough. Whip a bit longer at a low-medium speed so you don't end up over-whipping, for 15 - 20 second intervals and test again. Do this until the meringue sticks completely to the bowl when fully inverted (see step-by-step picture 4 above).