How to identify Ag nanoparticles in a solution?
This paper says that silver nanoparticles enhance the enzymatic reaction that happens in a normal ELISA assay. You should be able to observe a significative shift in UV absorbance comparing your bacteria to a control. 

Otherwise, I'd think simply of lysing the bacteria and centrifugating the lysate. I'd expect the pellet to contain nanoparticles and to be somehow different (in color?) from a control pellet.