There is a way of finding it u simply click an a a rattata,Pidgey,zubat,magikarp but don't attempt to catch it then you go on to the pokedex it will say how many you have seen if it says you have seen one that's where it's hiding

There is a way of finding it: you simply click on a Rattata, Pidgey, Zubat, or Magikarp, but don't attempt to catch it, then if you go to Ditto's page in the Pokedex, it will say how many you have seen if it says you have seen one more than before, that's where it's hiding.