For a Pokemon to be registered into your Pokedex, you must encounter it first. As far as my knowledge goes, if you see a shiny in the overworld, it will not be registered in your Dex unless you encounter it.
Unless you were recording, I don't think that there is a way to tell, just keep going!
Hope I helped, good luck getting the Golden Raddish!
I was Shiny Hunting in LGP with a friend, and we found a shiny Magikarp. He checked his dex before encountering to see if it was a duplicate or not, and shiny Magikarp wasn't there. Again, hope I helped! :]