how does the game know if its a shiny

1.)When is it determened that a pokemon is a shiny for your account? Do they spawn shiny, do they become shiny once you tap them?

2.) There is roughly a 1 in 450 chance for a pokemon to be shiny, would it help if you only tap pokemon that can be shiny? I mean, do you "waste" a shiny chance by tapping a non shiny(i.e. snorlax or any pokemon that is not yet shiny in the game)?