How To Inventory a Stockpile?
The z stock screen can indeed break it down by bar type. Make sure you actually have your "Bars" section selected, and your tab mode set to group items. In addition, make sure that your bookkeeper is working with adequate precision to view individual items. Here is an example from the DF Magma Wiki showing a breakdown of seed types:
Other than this, there is no way to view all contents of a given stockpile.