When you’re on the maps screen, you’ll see a tab in the bottom left with Pokemons and steps on it. This is your Nearby Pokemons area.
Tap it and you’ll see a list of Pokemons that are around you, and you can catch them. Each Pokémon has three feet below them.
Three feet means the Pokémon is far.
1 foot means the Pokémon is very close.
No foot means you’re literally standing at the Pokémon spot.
Also, in the Nearby screen, a Pokémon in full color means you’ve got that Pokémon. One in silhouette means you haven’t.