If “Double knuckle knock” becomes more than, I dunno, presumably some gross phrase you’d find on Urban Dictionary, you may thank the folks at Qeexo:
FingerSense is an enhancement to touch interaction that allows conventional screens to know how the finger is being used for input: fingertip, knuckle or nail. Further, our system can add support for a passive stylus with an eraser. The technology is lightweight, low-latency and cost effective.