differentiating proteins in/on bacteria - (Sep/06/2013 )

I have been reading an article on how they created a diagnostic kit to detect bacteria by using antibodies against antigens.
The antigens were proteins on the outside of the bacteria.

What I wonder is: how do you find these antigens/proteins on the outside of the cells?
How do they seperate the proteins that are on the outside of the bacteria from the bacterialcell (and content inside).

Is there some sort of system that you can use to easely collect outcell wall proteins or?

I can imagine you can collect all the proteins, by just lysing the cells or something,but not sure how you discriminate outer vs inner proteins (antigens).

(I did read/check some papers, but they seem to use statistics and the configuration of the proteins to make a distinction, so I wonder: is this the only way to do it? Or are there other ways?)

Presumably you could fix the cells and use them to immunize the animals, then take the resulting antibodies and use them to IP proteins from lysed cells.  The antibodies should only detect proteins on the outside of the cells.