I'm looking to track how many people on the web have made my software program file available for download without my permission.
I've thought of searching for my product name and file size to catch possible thieves.
Do you think a web search API is the best way forward?
EDIT: I plan to use the detection data for survey purposes.
There are web analytics companies which could probably help you more than a roll-your-own solution. Consider the companies that big music and film vendors use, or check with the Business Software Alliance.
Ultimately, you are chasing your tail if you are looking to thwart piracy with the results of this kind of activity. However, if you are steadfast that you must try to understand what is going on, you need professional (web analytic) help. There are so many variations out there that you need someone experienced in tracking this kind of information, since you could easily get a false sense of security, or an inflated sense of activity.