I read some messages about difficult spotting at CDG.
Well, I was there last WE. There is no chance to spot at the fences. The MP is patroling all the time and they will ask you very friendly to stay away from the fences, but they will only ask once. I asked if we could take pictures on the other side of the road. That's about 50-100m from the fence and the MP said that was fine.
So I ended up with great pictures taken on my 100-400 mm lens. But I miss the close ups from the taxi-ways.