In the movie, on a few occasions, we see characters who have a strange gold like glow in their eyes. This is meant to suggest to us, the audience, that they are replicants (or animoids, in the owl's case).

Note: The glowing eye effect was achieved by bouncing light off a half-mirrored glass mounted at a forty-five degree angle on the camera. An effect that is quite difficult to create (and even harder to accomplish by mistake...)

But note also that the owl was originally intended to be real - Rachael's line was overdubbed to indicate the owl is artificial.

If their eyes are glowing, why the need for V-K tests?

Surely anyone can see who is a rep and who is not... [Well... not really!]

The use of the glowing eyes is a narrational device, a cinematic technique employed by the movie's makers to tell the audience who is a replicant and who is not. This "device" goes outside of the movie's reality, meaning: we, the audience, see it, but the characters in the movie *do not*. Hence the importance of the V-K test, which in the story is the only way to tell a replicant apart from a human.