By his coat of arms, usually. Heraldry was a major factor in medieval Europe, and precise identification of the crest was vital.

Word of mouth was also important - many people should know the knight, whether their relatives, seniors and vassals. In the modern day we rely on photographies or TV, but getting to know people was incredibly important back then. Foreign envoys who were outside the society and usually (but not always) couldn't count on recognition, carried objects marked with crest of their monarchs.

If a commoner wanted to impersonate a knight (and it's a bit like a Catholic impersonating a bishop), he could maybe fool other commoners if he was lucky or smart, but without a coat of arms known to other knights or at least a withess, he would have little chance of avoiding recognition.