Oryan77
Adventurer
I thought I was clear in my first post when I said "If a player looks up spells in the PHB just to figure out what spell the DM cast". I guess I was being too broad, but it was clear in my head when I wrote itHypersmurf said:This is the first time in the thread that you've included "their PCs have never encountered" as a restriction on what rules they're prohibited from looking up.
This was the information missing from everything you've said so far.
Prior to this post, your policy read to me as throwing the baby out with the bathwater. The PHB contains information the characters don't know, therefore the players can't look anything up.
You've since clarified that you don't mind them looking up things the PCs do know.

What I should have wrote was, "If a player doesn't recognize a spell an NPC cast & he looks up spells in the PHB just to figure out what spell the DM cast?"