Find out how the competition stays in business.
Bar graphs
Web Browsers (GRML)
Web Browsers (GRML)
Web Browsers (GRML)
Bar graphs
Web Browsers (GRML)
Web Browsers (GRML)
Web Browsers (GRML)
Web Browsers (GRML)
Bar graphs
Web Browsers (GRML)
Web Browsers (GRML)
Web Browsers (GRML)
The first thing you're going to want to look at are the backlinks. This will show you if there are other websites where your competitor is advertising or where it's receiving free traffic.
Also, many websites have public website stats available (whether they know it or not). Search the source code of the website and look for a tracking script, then try to login to the webstats program with the username from the competitor's source code. Several free traffic stat programs don't require passwords if you get the username from the source. The traffic stats will probably offer a wealth of information regarding your interests including referral information.
You could also check out the Alexa Rankings for your competitors site. I don't really trust the Alexa numbers, and it won't show you exactly where your competitor's traffic is coming from, but it may give you some idea of how much traffic they're receiving, and where else your competitor's visitors may be going.
Lastly, take a look at the pay per click engines, and see if your competitor is advertising there. Overture, Google, Findwhat, etc. If your client is paying for per click advertising, then you can be certain they're receiving some traffic from the engines.