To evaluate a job-site. I try to apply to it as a candidate to see how friendly and accurate it is in parsing the information and whether it is easy to fill in the information.
After that is done, I try to do job searches and set up alerts to see how accurate they are. Are they just spamming me on key words or are the matches truly accurate.
This gives a fairly good idea of a job site. Because a job seeker is looking for jobs at multiple sites and the last thing he is looking for is filling up the same form again and again on multiple sites. He does not want alerts sent to him just on the basis of key words but more accurate alerts based on algorithms and combinations.
A good jobsite is able to attract passive candidates - that is when as a recruiter I would partner with the site.