Notice if bird poop falls on your shoulder or around your waist. In reality, the bird poop is likely a concoction of several condiments. Scammers aim for these areas as they are likely to carry the straps of a purse or backpack.
Exercise caution when a random person offers to help you clean off the mess. The person will have a napkin or handkerchief handy. His actions are supposed to encourage you to take off your purse or backpack so that he can reach the spots that the backpack or purse covered.
Note aggressiveness. The scammer may insist that she try to help you clean the bird poop off or may try to slip off your purse or backpack straps. Know that she is only trying to take your purse or backpack off so that she can run with it as soon as it's separated from your body.
Walk to a safe place with the supposed bird poop on your body and clean it off when you get to your destination. In order to take advantage of you, scammers rely on your disgust and your desire to get the poop cleaned off quickly.