Well, the request they send would contain that text. For example, they'd say ""GET /robots.txt...". Their request usually contains other information, such as the IP they're using (or claiming to use), the name of the browser or user agent, and so on. A user agent might show up as "Mozilla/2.0(compatible; Ask Jeeves/Teoma;+http://sp.ask.com/docs/about/tech_crawling.html)". This is a polite bot that contains an address where you can get more information about it.

Of course, someone could fake most of that (maybe all of it), but they usually don't. And anyway, even if it's MotherTeresaBot, if it's hogging your bandwidth, it's still causing you problems.