As someone who DMs DnD 5e:
Um. No, you are not allowed to look in the Moster Manual!
I get the idea though, and I guess if it's tied to some form of skill check, then it could be cool.
The reason most DMs don't allow people looking up the monsters, is so the players won't power game the heck out of encounters.
On the other hand: Wouldn't most people know at least something about Goblins? They are everywhere.
