Click anywhere, or press Space, Z or X to do an encounter. This generates a new Shiny Rank. If your Giratina's Shiny Rank is 1, the Pokémon is shiny. To indicate that an encounter happened if you're not sure, Giratina's cry will play, and there is a 50% chance that it will switch from its altered form to its origin form and vice versa.
Type "shinytest" to automatically encounter Shiny Giratina. (this does not update Shiny Encounters) Type "resettotal" to reset the Encounters variable.