If the compound potassium chloride is written in words, how do you know to write the formula with one potassium and one chloride

Because there is only one stable ionic compound made up of potassium and chlorine, and that is KCl. So calling is "mono chloride" or similar would be redundant assuming you understand basic chemistry (i. e. knowing oxidation numbers of K is + 1 and Cl is - 1). When compounds can exist in multiple forms in nature like CO and CO2 you will preferably indicate it through the nomenclature, calling one a monoxide and the other a dioxide.