Could a player, during their turn, count the number of resources of each kind in the bank? I would typically do this before playing a monopoly card for instance.
You can only count resource cards after rolling, to determine if there are enough for everyone to get the cards they need.
From the official FAQ:
Resource Cards - May I count the Resource Cards in the supply stacks during play?
Yes, immediately after a production roll, before distributing the resources - but only if it isn't sure whether there will be enough cards for all players entitled to receive them.