The basic answer has already been given by Zikato: Falcon has goggles as part of his suit that apparently include enhanced targeting protocols and an advanced Heads Up Display (HUD).
Promotional image of Falcon showing goggles as a part of his suit while he confronts Ant-Man before Ant-Man shrinks:
Here is a screenshot showing Falcon's POV while trying to locate Ant-Man's signature on the roof of the Avengers warehouse.
And here's Falcon's POV while his goggles have located and locked on to Ant-Man's signature, and zoomed in to provide a better image (Ant-Man is pictured running, roughly in the center of the "zoomed" window in the HUD):
once Ant-Man has destroyed Falcon's suit from the inside,
you can see he's no longer able to track Ant-Man via his Falcon Goggle HUD: