In the day the sky will be clear blue
In the night the sky will be black/blue
At Sunset the sky will be orange/redish hue
You can also check by evolving a Pokemon.
Max out Eevees' s friendship and then level it up. In the Day it will become an espeon and in the night it will become an Umbreon
Espeon - 4am to 8pm
Umbreon - 8pm to 4am