I heard you can do some kind of trick by sending them an email with a fake image in it, and then faking the http response and getting their IP (and other info). Is there any easier way?
Using this type of trickery is not the purpose of these forums. Discussion should be limited to what is the correct manner in which to do something like this, but not by these methods. Using methods like these will surely make a rival's customer stay away from your site.
One important fact to remeber when working on the Internet: "If you put it on the Internet, it stays on the Internet." What you are trying to do is akin to opening a Super Wal-Mart and trying to hide it from all of the shoppers from K- Mart.
If you have products that are sensitive, either don't put them on the site or copyright/patent/trademark them for protection.
all it would take is one of the said company people to use a dialup service, one of the many that are available and then access your site via that dialup which will be an IP address not in the same range as the company internet.
They can also use one of the thousands of free proxyservers that allow the internet users to browse other site without being hindered.
Both methods work fine for instances where you are unable to access a website because your IP address or DNSname has been barred by that site.
So save your self the bother and get on with your site, your competitors have better things to worry about than spend all their time looking at your site, so why are you worrying?