Can you look at a Bonus Marker before placing it on the board?
My group has always played that you could look, but while digging through the rulebook to support that decision, I've come to the conclusion that the opposite is true and that you cannot look. 

From the English rules:


  For each bonus marker a player obtains during his turn, he should draw
  a new bonus marker from the supply and put it face down on the tin
  plate on his player mat without looking at it. Before the next player
  starts his turn, these bonus marker(s) should be placed next to any
  route(s) of the player’s choice that meet the following three
  conditions: (i) no bonus marker is already on that trade route, (ii)
  no resource is located on that trade route (i.e., all fields need to
  be empty) and (iii) there is at least one available (i.e. open) Office
  space in an adjacent city to the route. 


You could still argue it's up for debate since it doesn't explicitly state that you cannot look at it before placing it, but based on the fact that you can't look at it when you receive it, and it never states that you can look at it, I think the implied answer is that no, you cannot look.