Aperture’s Highlight Hot Areas command (which you can access from the View menu, or by pressing Option-Shift-H) activates a highlight clipping display in the Viewer pane. This display shows any pixels that have clipped highlights. You activate this feature by choosing View > Highlight Hot Areas, or by pressing Option-Shift-H. Aperture will display any clipped pixels as red.
With clipped highlights displayed you can then use Levels or the Exposure Slider or the Brightness slider to darken your image until the clipped areas are gone.
However, if you’ve used the tool much, you may have noticed that it will sometimes indicate that there are no clipped pixels, even though a tiny spike remains on the right side of the histogram. This behavior is not a bug, it’s actually by design. If you look in Aperture’s preferences, you’ll see a slider labelled Hot Area Display Threshold. This controls what actually “counts” as a clipped highlight for the Hot Area Display. By default, this slider is set to 94%. You can’t set it to 100, but you can move it up to 99% if you want a Hot Area Display that corresponds more closely to the clipping that you’ll see in the Histogram.
The idea behind this control is that not all printers have the same white point. Some actually max out before the white that is displayed on your monitor. So, Aperture allows you to change the clipping display threshold to more accurately reflect the white point of printed output. For most instances, the default behavior will be all that you need, but if you want to make extremely refined adjustments to the very brightest highlights in your image, then you might want to change this parameter to a higher value.