Lisa Hirsch has written an article about how concert music critics prepare for concerts. It's very well done and the information in it tracks with my experience. The short answer to how a critic prepares for a concert is this: It depends. It depends on the program, the performers, the occasion--the variables are numerous. I'm finding my preparations for the current Tallahassee Symphony Orchestra season of music director auditions unique, in that the music is, for the most part, the Warhorses of the Apocalypse. I know most of the music fairly to very well, so I am able to concentrate on the various parts of the conductors' work.
I enjoy reviewing new music the most, though that has its own pitfalls. The greatest of which, in my experience, is not knowing for certain whether the performance of a new work has been a good one. And what does good mean in that situation? Accurate? Putting "more" into the piece than the composer meant so as to make it a better piece than the score? You might be surprised how often this happens when the composer is present at some rehearsals, especially if the composer is young and/or inexperienced.
At any rate, thanks to Ms. Hirsch for an interesting discussion.