[bigbugbag]

Either you just know because internet or you get it from trial and error.

It's probably written on recaptcha website or wikipedia article, but basically you get presented one word that machine can't read (the distorted letters) and something out of a book that google had digitized and uses you to ocr it. You also get a lot of address numbers for google street view.

Just input the distorted word and type anything for the other, and it will work. Of course the instructions won't tell you that or else people would act accordingly and google would lose this free labor source for the tedious work of proofreading digitized version of books.