The biggest thing I'd say is to look for the color difference, because I finally got one and I didn't see any stars when it appeared, but it was a Venonat with blue eyes instead of red and I was like-HUH? That doesn't look right,lemme catch it quick, and it was a shiny indeed! The outline that's kinda pale purple on the others when you look in their summary was blue instead and he had a star in the top right corner of his picture. When he comes out of the ball when a battle starts you can see stars, but I didn't before I got him. I found him in the grass right when you get to the path towards Bond Bridge on Island 3, but I don't think that has anything to do with it, they pop up randomly where ever as far as I know. Good luck!
I got all the Pokemon a person could dream of, and still working on more. If you need to know the truth, seek it out here, as I am obsessed with answering all the questions I can as best I can. I can verify Event Pokemon, too! My collection keeps growing!