While Niantic hasn’t mentioned anything in the latest update, users are posting screenshots of the shiny Pokemon they caught.
According to the comments, the only way to tell if they are shiny is by going to the capture screen. It doesn’t show on the map and some users also reported their shiny Magikarp turned in a normal Ditto after capture.
As mentioned before, the only shiny Pokemon spotted so far are Magikarp and Gyarados, it may be related to the ongoing Water Festival that will end in March 29. If you’re after these, better hurry and try to catch one before the festival ends, just in case. There may be more shiny Pokemon in the future but, due to the lack of official updates about it, at the moment we don’t know.