It is caused by the airflow on the top surface of the wing reaching supersonic speeds. The airflow behind the subsequent 'shock wave' becomes turbulent, rather like pre stall buffet. (Simple answer).
The indications are much like pre stall buffet, except perhaps felt at a higher frequency. You feel it just as you feel the low speed buffet.
Much like low speed buffet, it is a function of loading. The higher the coefficient of lift, the sooner the buffet will appear. So if you are heavy or manoeuvreing, the high speed buffet will appear earlier.
To my knowledge, no stick shake devices which warn you of high spped buffet have been fitted to aircraft. However, on one aircraft I flew, we used to bug the high speed 1.3g buffet boundary on the ASI at higher altitudes, and on another with a glass cockpit, the high speed buffet boundary was indicated by a series of red bars ('red bricks' as we knew them) much as the low speed boundary was.