The easy way to know, if you are booking online, is whether you select any one of the "special rate" radio buttons (AAA, Senior, Govt, Corporate / promotional code). If you do, you are requesting rates that you may need to provide proof of eligibility. If you go another step further and enter a 3 digit code into the "Corporate / promotional code" box, you are even more likely to be challenged, as you are now in clarkef's example. (assuming it's either a "random" set of digits trolling for cheap rates or a specific company's 3 digit code... there are plenty of "public" codes one can try, see the Sticky)