Plain old straight forward User Testing should reveal how effective your recommendations are. Just establish a set of criteria for threshold acceptability and run a few trials to see if your system reaches these limits.
The key here is trying to determine acceptable success criteria that directly relates to the value you're trying to add be incorporating this feature. If it were my test I'd want to know:
What percentage of users noticed the recommendations
What percentage of users followed the recommendation links/prompts
Of the links/promts followed, where in the displayed list did they rank
You could get even more dense with your criteria (such as how long did it take the user to find and act upon a recommended link), but unless you're planning on testing a fairly large sample the value of this level of resolution is questionable.