I have been photoreading biographies lately and a few of them didn't have any subheadings or real chapter titles. In those cases, I've found it useful to extend my previewing just a touch and instead of just looking for key words, I look for key sentences. Trust your brain and let it pick and choose. Flip every twenty or thirty pages, scan the page, and let it pick up a sentence or two that tells you where the person was at that point in their life. I spend literally about four seconds on a page, but it's useful. Previewing is just giving you a map of the territory. The map is not the territory, but it gives you an outline of what to expect next. It gives you a place to start forming and asking questions. Basically, just try to get a feeling for what the time periods meant or what major events were happening, and if you cant figure it out then you have a perfect question to ask. Just ask your unconscious mind to fill in the blank and you'll be surprised by how often it does way more than just fill in the blank.
Best of luck.
