Does the pokenav app that tell you when you've found all the Pokemon in an area?

Yes, the DexNav lists all of the Pokemon that you can catch in each area, including the shadows that you have to sneak up on, and any Pokemon that only show up in hordes.

If you need to know if you've found every Pokemon in each area, a crown emblem may appear in the upper right of the DexNav, to indicate the player's progress.

• Platinum: all of the wild Pokémon in the area are in the player's Pokédex;
• Gold: the only missing Pokémon are foreign species unlocked by the defeat of Groudon/Kyogre;
• Silver: the player is missing Pokémon from only one of the other encounter methods (tall grass, fishing and surfing);
• Bronze: the player is missing Pokémon from two of the other encounter methods;
• No crown otherwise.

Source: Bulbapedia.