depends on who it is and what it's for. for a big event (wedding etc) where i have to formally cater, i chase them up - and make sure i make them feel guilty - ie "seeing i haven't heard from you and i need to finalise numbers, can you PLEASE let me know what's happening".

if it's something less formal (like a kids birthday party etc) i ASSUME they're not coming - but i always over cater anyway. we have things like sausage rolls in the freezer and chips etc in the cupboard - if people come that we weren't expecting, it takes nothing to take extra food out. i won't cater extra if it's stuff that has to be used straight away and can't be frozen though (so for bbq, we buy from butcher that morning - cook what we need, freeze extra cos we can use it later)