On Teslas there is no handbrake in the traditional sense of the term, i.e. a lever to pull. There isn't even a button of the type increasingly found in cars equipped with an electric handbrake. But the "handbrake" (or more properly parking brake) is there. It's just a little hidden.
By pressing the button, the car normally goes into Park, and this is usually sufficient for stopping and parking. To activate the actual parking brake, however, this same button is used, holding it down for a few seconds.
When properly applied, the classic parking brake icon appears on the dashboard.
In the photo below you can see it on the left, near the upper edge of the display.
From the Model S manual it appears that this brake can also be activated via the touch panel, is independent of the brake operated by the pedal, acts only on the rear wheels and cannot be released if the car were completely without electricity.