Interviews are a two-way street. They learn about you, and you learn about them. Give or take your willingness to believe glassdoor, you have two opportunities to get information about a possible employer. You might know someone who works there, and you can learn from your interview.
There are two ways to learn from an interview. You can ask questions, and you can draw conclusions from what you see. Asking questions you can figure out for yourself. Observing ... Hierarchical companies will have very different processes than flatter companies. If you are only get to talk to managers, or you the worker-bees that interview you are working from a very tight script, that's some evidence of hierarchy. Or if people refer to management as 'Ms. Foo' instead of 'Grace'. If you talk to a wide range of people who ask you a varied collection of questions, that's evidence of flat. If they ask you questions about how you can contribute to a flat process, well, there you go.
You will also learn from what you see. Where do people sit? Do they hang out together in common areas?
Just about every company that is actively recruiting will make the same set of claims about employee empowerment, yada, yada. So reading the
propaganda web site isn't going to teach you very much. Even a company that makes very specific statements about 'flat structure' may be merely reflecting the conceit of the HR department. Also, keep in mind the structure in which all is flat except for one person who is in charge, period. That person may flatter her or himself about running a flat company, when a better term would be 'flattened'.