While I regularly make it a habit of observing other pax on a flight (mostly out of curiousity - I like to people watch) - I would never ask someone directly if they are an Air Marshall. To me, that is similar to "outing" an undercover police officer - their job is to be there incognito, so I would hate to put them in a compromising position by asking them to identify themselves. Even if this discussion takes place in private, we know they are supposed to be undercover, and not allowed to reveal any info - so I personally would not ask.