As part of a promotional campaign, my company wants to launch a site where users (and potential users) of our product will be required to register and vote for certain choices. Depending on how successful it is this might become a regular feature. Since the proposed prizes will not be insubstantial, naturally we are concerned about voter fraud. How can we detect the usage of proxies, Tor and various other methods that might be used by unscrupulous users looking to game the system? For example, how does https://check.torproject.org work? Are there any ready-made scripts we can use, not just for Tor but in general?
I see that Stack Exchange has held competitions in the past based on online voting, so I am hopeful of getting some good answers here.