I'm trying to spy on a competitor's website and I get that from 100K visits a month he has several destination sites, but when I go to his website he is actually promoting just one product, for one destination site. How is that possible? Does he have different landing pages? How can I find that out?
Depends on what you are using to track his keywords or general traffic. Some services give a lot more information than others (usually the paid ones), and some can make relational links that will help you get to the bottom of this situation.
What are you suing at the moment to dig up data on this site?
You could type in the competitor's domain into linkpopularity to see who is linking to them or you could just type in the domain name into a search engine to see what kind of doorway-pages they have indexed
similarweb. com is really cool. The full information in a short time.