Not "know" in the sense of coming up with a name for it and being prepared to pigeonhole it as this or that (or getting distracted when it doesn't see to fit into any familiar category). More like: notice what you're experiencing (including what you may be telling yourself about it). Something like, "That cloud... I don't have a name for shapes like that but one end reminds me of the back end of an elephant and the other, of Little Red Riding Hood's head with the hood up."

Perhaps the most important "ground rule" of meditation/mindfulness is to take what you get and not dismiss it or argue with it or make excuses for it.