Dungeon Master Encyclopaedia

When you look at a potion, you can only see its power level if your champion is at least a Novice Priest (Skill level >= 2 in skill #2). The power level of the potion is shown with the power runes that are also used in spells.
Note that there is a bug in the game that will display an underscore character "_" when you look at an 'Empty Flask' (this one was fixed in CSBwin) or a Potion with power < 40.