Surest sign of a long penis is its owner's need to announce that fact to the world. 
(via Instagram)

When 23-year-old Anna Gensler, an artist, joined Tinder six months ago, she was hoping to meet guys. Instead, she was matched with a whole bunch of creeps. Their opening texts ranged in message and tone from "I have a long penis" to "Bet your [sic] tight." Gensler was—like most women on dating sites—put off by the dudes, who seemed to think these disgusting overtures would actually work. 

(Images below slightly NSFW for poorly drawn hint of penis.)

Sources: Slate | Instagranniepants on Instagram