I work for a charity and would make a couple of points.
Firstly, you can always contact the charity that the bag pupports to support to check if it's genuine. The bigger problem, as some previous posters have said, is sharks bogus collectors going round and collecting the bags before the real collectors arrive.
In terms of how much money goes to the charity, obviously it varies. However, I would caution you to bear in mind that although 100% of the proceeds may go to the charity who does their own collection and only a portion goes where it is outsourced, both have a very similar level of costs to ultimately meet.
So, the charity that does their own collection will then have to pay themselves for the staff, vans, insurance, the sorting etc, etc. The charity that outsources will receive a donation that goes directly into their charitable spends. Very often, there is little difference between the two options.
I have to say, this was something that surprised me when I first got involved a few years ago. Now, I am very aware that the bogus collectors are the ones we need to be watching and that the genuine ones all make a significant contribution to many deserving causes whether they are done directly by the charity or outsourced.