You'll know it when it happens, there isn't anybody willing to deny that.
You'll be walking, you won't be running. And you won't be in a car because then you're just seeing her on TV and even if you forget you know she's just painted phosphors and glass. You'll be walking for certain. And all that is happening will twitch for a second. You'll take. Double take (if you took the first time, like you were supposed to) and still... what was it you saw? Like skimming a stranger's book case in a hurry... "The Old Man and the Grapes of Wrath"? No such book exists––right, a trick of the mind. What was it you saw? "A Traffic Light with a Silk Scarf"? No!
You'll then cease walking, lips cracking with a thirst.
It will take you roughly a minute. To inventory everything on the scene, careful not to combine any objects (or rather, the titles of any objects––a matter of the heart here). Then you will notice that, in a kind of passive meta-retrospect, whatever it was was absolutely beautiful and that it is now gone––whatever it was.