Pry for information. Don't accept generalities. Ask specific questions to get specific answers. How specific depends on you, your beta reader, and how intense you want to make your beta reading. I'm usually interested in an objective viewpoint of the story and characters especially. But sometimes, I want to know about a certain scene, description, or section of dialogue. If you have a particular section of the story you want feedback on, ask your reader to pay special attention to it.
Listen! I believe one of the keys to a successful beta reading is not shutting down mentally when your beta reader is honest about something you need to fix. They could very well stop giving you much-needed feedback if you get defensive or act hurt when they tell you like it is. Remember, a beta reading is to improve your story, not just receive glowing accolades. Those are wonderful, but won't really help you to reach your goals in the end. This is easier said than done. But swallowing your pride and getting down to business with your story is the best thing you can do.
An objective reader is a blessing. It's hard to see the forest for the trees, or vice versa, after days, weeks, or months of writing and revising. So keep your objective readers close at hand. They will help you improve your current WIP and works to come.
Photo by Toms Baugis