They sometimes weigh the hand luggage at check in if you are on or slightly over the hold luggage allowance.Helped us as the hand luggage was over but the hold luggage was under,so we were let off!.Haven't personally seen any weight checks at boarding gate but have seen bags being taken off passengers at this point and put in the hold.
On our last holiday,a group of people were arguing with check in staff ,who had said that they were going to have to check a couple of big holdalls in.About time that they did stop these grossly oversize bags being allowed as cabin baggage,in my view!!