Good question and it's the one everyone asks. There's a couple ways you can figure this out. Scrapebox lets you scale the process of looking at google suggest. You could probably build a scraper of quora suggestions. Anywhere people are asking questions about topics related to your business will help you figure out what content. There's a google docs tool made by a guy named Richard Baxter that does something similar I'll see if I can find it and post the link here.