There are several tools you can use to evaluate your site’s performance. You can star by analyzing data from your standard analytics tool (Google Analytics, Omniture, etc). In addition, I would recommend using analysis tools that help understand on page behavior (Clicky, Crazy Egg, etc). If you want to better understand content usability and expectations you can conduct an online card sorting exercise using relatively inexpensive online tools (optimalsort.com, websort.net, etc). You could do a survey of your users but this usually not very effective. Finally, you can conduct a more advanced eye tracking study but I would only recommend this investment only if you are building a major enterprise level website ($400k to $1m).

So, the standard approach to evaluating site performance for purpose of web redesign is as follow: 1) gather the data/information using the tools above, 2) conduct a competitive/landscape analysis and 3) use a digital strategist to interpret results and make specific usability/design recommendations.