In many cases, wanting things in game terms isn't breaking character.  To work within the stated example, a character wants to know how difficult a fence is to climb.  Well, fence can mean a lot of things, from a knee high string of white posts, a chain link fence of normal size or a concrete barrier with barbed wire on top.  

In every scene the GM has to supply all the information for the characters senses, but you can't do that because describing everything in full detail would totally overwhelm the players.  So it's a delicate balance of trying to give all the relevant details but not any unnecessary ones.

In addition you've got possible differing ideas between players and GM.  If the player thinks scaling a chain link fence should be an easy task and the GM thinks it's a hard one then since the GM's interpretation is the one that's going to win, the player has to ask.

And there's also cases where a character is good at a skill but the player has no clue, and so a description doesn't tell him how his character could evaluate the challenge.  

And that's not to say there aren't also situations where there character shouldn't ask, "How tough is the skin of that enemy I haven't attacked and so have no way of knowing?" for example.