Is there any way of knowing how many bags came with a set? I have just acquired 9474 Battle of Helm's deep and have some others where the box is open and I don't know if all the bags are in there. For example this set it looks like bag one has been opened, and I have three bags each of 2 and 3.
Instructions
If you cannot find a review, most sets detail which bags you will be using for each stage within the included instructions. LEGO provide a easy lookup for most instructions included in sets if you do not have a physical copy.
Online reviews
An easy way to find out these sort of things is to find a review online, for sets of this size there are often quite lengthy details.


A total of 9 bags, each with 1 or 2 smaller bags inside.

Weight
To be sure you could also weigh all the bags, add them up and deduct it from the known weight (subtracting the box and instruction weight). Physical information of sets including dimensions and weights can be found on LEGO set databases like Brickset.com