But you can! And there's pretty much nothing Blizz can to block players from seeing amount of nodes unlocked in your nearby target's currently equipped artifact.
- Take GearScore as baseline. Or Exorsus Raid Tools which can inspect raid members.
- Get base HP for target's class and level from the table.
- Inspect target's gear, calculate stamina -> HP from gear.
- Divide target's actual HP by (base_HP + gear_HP), you get 0.75%*X where X is amount of artifact nodes unlocked. (or subtract, w/e, you get the idea)
- ???
- Profit!
I'm not familiar enough with Lua and Blizzard API to code this, but for an experienced addon author should be easy. I ask you to do this ASAP, it's very useful for all forms of non-random group play.