Easiest way,
head to a shop and check the small shield / circle in the top right corner of an item.
Bronze has a quarter filled circle, Silver a half filled circle, Gold a 3/4 filled circle, Platinum is completely filled and Niob is a full yellow circle.
Also, healthpotions do less healing in higher difficulties. Thus when it shows 100% it's bronze, when it's 70% (I think it is) it's Silver.
Thorin