okay so I see he used 8 spells without drinking a potion but what does that say me for the next encounter? Absolutely nothing, he might have sold the item or give it to an ally or get another mana item to be able to cast 20 spells in a row. Your information on the enemy mana pool is always totally outdated.
Right now Items which give mana are the ONLY items which give no clue at all away :
Health and regen : seen on the healthbar
Movement Speed : just need to take a look on the screen
Armor : take a look at your damage numbers
Items with special effects all or at least most, not sure if there are a few exceptions, have some indicators as well either on the UI or just as visual effect.
P.S. : don't try to lecture me on what skill is 