UX expert, author, teacher, mentor and true believer in the power of combining UX, data and analytics to prove that good design is worthy of your investment, but only if you know where to look, what to look for and what to do with this valuable information once you find it.
This is a large, important question with no single or simple answer.
Data and data analysis is key, as well as tracking what users are actually doing on your website. This can be done using Google Analytics and very inexpensive tools to see what users are actually doing, such a Crazy Egg. To do this correctly and effectively, requires having someone on your team with UX expertise who has done this kind of post-launch customer/user tracking and analysis and has acted upon the findings with measurable results.
Bottom line, the simple answer to your question is to hire a As someone who does this work every day, I will tell you that there are no magic answers. Do it well and your website does everything you and your customers/users want and expect it to do. Do it not so well will be forever confusing and you'll just be guessing and not knowing whether anything you are doing is doing anything.
My advice is to take the plunge and hire the right UX pro and realize the full potential of your investment.
Happy to discuss further!