Great Question. One thing you can do is search the recent sales section of top domain selling sites such as sedo.com, afternic.com, eBay.com, etc. For example, on eBay you can check the completed items and sort based on price (low to high, high to low, etc.)

Looking at the recent sales information can give you a general idea about the values of domains. Some of these sites may also have a sort feature where you can sort through to see the highest selling domains.

Also, if you are interested in making money online with domains, there is a great online guide that you can read for free below: http://www.yezzer.com/make-money-online-with-domain-names/

Have a great day.