Probably simple "Untouchables" point

Just watched DePalma’s “The Untouchables” again. Pretty good movie and all that, but it reminded me of one point that I never quite got. At the end, when the judge switches the juries in the Capone case, the following (parphrased) dialogue takes place.

Well, was it or wasn’t it? Did Ness just assume the judge was crooked even though he didn’t have any proof?

I am probably making this way more complicated than it need be, but any help for my feeble little mind would be appreciated.

My take on it (and God knows I’ve watched the movie at least two dozen times) is that the judge was crooked - that most of the judges were under Capone’s jurisdiction at some point - and that Ness just made a lucky guess.