In my last post, I mentioned sending a draft of a story off to beta readers, and being full of gratitude for their pointing out a flaw in the story that had never occurred to me. In the comments, Ali asked if there was a certain specific question that I always asked of my beta readers, some kind of feedback that I always wanted. The answer was long enough, I thought, for its own post.
Which is odd, considering that I could easily answer the question, "No." No, there isn't one single question that I always ask my beta readers, other than the implied, "tell me where this sucks." But she wanted to know about how to get useful critical feedback, and that is a longer answer.
The most important thing about beta readers is that they are people you can trust to tell you where you are going wrong in your story, and to tell you that in a way that is useful to you as you go about revising and fixing it. If everyone you give your drafts to always says "Oh my God, this is amazing. Publish it now so it can will all the Hugos," well, I hate to break it to you, but you need new betas.
But beyond that, you should ask the questions you want to have answered. If, as you are rereading your draft, you notice that the opening is weak, or you're not sure if the ending is earned, or if the murder in the second act is adequately set up, ask your readers to pay attention to those things. If you're writing outside of your comfort zone - a genre you've never written in, or a character of a gender or race or sexuality different to your own - ask your readers to comment on those things. If you know you wanted your story to address a particular theme, ask if it did.
Have different readers for different types of stories - don't ask someone unfamiliar with the tropes of romance to read your satire of romantic tropes. When I'm working on a novel-length project, I always ask friends who aren't writers to beta read for me, because I want the reactions of people who are going to read like readers - like what I hope most of the people who read the final product will be.
Don't waste your beta readers' time. It's one thing to send out a story and say, "I've looked at this over and over and I know something is wrong with it and I don't know what that is, please help," and another to fling an unread hot mess of a draft at people and expect them to do your work for you. Make your story (or chapter or novel) as good as you can before asking people to seek out its flaws. Tell them thank you when they get back to you, even if it turns out their feedback is not immediately useful. Especially tell them thank you if they have critiqued a novel-length project for you. Don't be afraid to ask questions if you don't understand why they gave the feedback they did. Don't expect to be graded on your effort - beta readers act in the service of the story, not your ego. Don't feel that you have to take all of the suggestions given to you - often enough, they will be directly contradictory, and then you quite literally can't - but do consider each of them seriously, and remember that you write in the service of the story, not your ego.
Don't be surprised if you get answers to questions you never asked, and those were the most helpful of all.