Suppose you play Pokémon with the sound off, and without paying much attention to the screen. Suddenly you notice a wild Pokémon battle has started. You think it might be a shiny, but you didn't see or hear the sparkle at the start because you weren't paying attention.
Worse, you don't have Bulbapedia to hand, so you can't look up the sprite for comparison. You don't have any cheating devices either.
Is there another way of determining if that Pokémon is shiny?