Building That Affiliate Site

As discussed in my previous post, there are a lot of MFA(Made for Adsense) sites abound and not as noticable, made for affiliate sites. The question that arises is how do you know what type of product to focus on, when building a site for the sole purpose of earning revenue off of affiliating. The answer is not simple and does not come quick, so brace yourself...'A lot of research'."A