On an individual basis, you can’t. That’s the reason for doing it. But en masse, if there’s a whole bunch of people complaining about the race of the cast before it airs, and then immediately after it airs a whole bunch of 1-star reviews appear like clockwork, then it’s not hard to draw the line between the two. It’s not the first time people have review-bombed things they don’t like, and it won’t be the last time. All they can do now is complain “how dare you not let me review bomb this show!” until the next target appears.