There are a number of good resources available to listen for relevant information or discussions about your brand. Companies such as Radian6 offer quality tools, but not everyone needs to invest in such a system.
Some of the easiest things you can do are review your Google Analytics or social media analytics available to you for free. Sites such as Facebook and YouTube both offer supportive tools to help you understand what is working and what is not.
Google alerts is another great option to keep you up to date on the conversation about your brand.
The folks over at small business trends have a nice list of twenty free options.