They have a lot of landing pages. You can see what types of pages they are producing by searching site:domain.com in a search engine. For example, if I wanted to see how many landing pages Moz has created targeting long-tail keywords, I could search site:moz.com and get a sense of how many landing pages they have. Check out Alexa search rankings and look at the percentage of visitors your competitors are receiving from search. If you look across competitors and one site receives a lot more visitors from search than others, you can guess they are using some kind of SEO strategy.
