If you can attack them successfully then they're weak. If not, then they're not weak.
You attack them by maneuvering pieces to attack them.
Not very helpful I know, but it depends on the position.
Plans need some kind of followthrough.
Sometimes it's easy. I attack them so _____
Answer: so I can win material. It's easy because being up material is generally good.
Sometimes it's not as easy. I leave them to block enemy pieces so _____
Answer: so I can attack an area faster than those stuck pieces can be organized to defense or counter attack. It's not as easy because you can't be sure (calculate it to the end).
How do I take advantage of weak doubled pawns? Do I attack them or leave them to get in my opponents way? Actually, a better question is whether anyone can post an example of these two situations. I think either is a valid idea, but I would rather have input from good players on this.
Also, I hear that black can't get away with as much in the opening as white. I don't mean this in the context of studying openings, I mean it in the context of opening principles. Is this true, and is it a good strategy as black to try and block everything that white attempts, or is it better to go for the neck and attack?