If in doubt, ask.
I've been vectored off flight planned track in IMC over elevated terrain and cleared to descend to A030, grid LSALT 3800. I simply read back the clearance and appended "Confirm, radar terrain cleared". The controller confirmed and away we went.
Quite often controllers will automatically include the term "radar terrain cleared" (or similar) with descent clearances, especially when the aircraft is over, or has just left, an area of high terrain in IMC or at night.