How do I know that Thunder I/II is doing something?

[Cabalabob]

Potency is a % multiplier, you have a base damage which is determined by your weapon/magic damage, magic attack(same number as your INT) and your determination. Once you do the math to get the number which I'm not going to get into cause you don't really need to know it. Let's say you get 500 as your base. For now ignoring crit and astral fire. If you used a 200 potency attack you would deal 1000 damage (500x200%). Give or take anywhere up to 5% due to the random number generator which makes it so you don't deal exactly the same damage every time.

On topic, you don't really need to know thunder is doing anything, at higher levels Thunder isn't really used for the DoT damage but for the chance at thundercloud procs and believe me you'll notice those.