Humans, after they have obtained the eyes of a Shinigami, can then see the names and the life spans of other humans and their names. Which only then means they need to envision the face in order to kill them. However, humans with the Shinigami Eyes cannot see their own life span nor that of another human that has gone through with the 'eye deal' or obtains a Death Note. In Death Note: How to Read 13 it was revealed that a human with Shinigami Eyes obtains 3.6x their normal perception.
In the Death Note Manga the eyes are described as being a Golden colour with bright red pupils and a pale yellow sclera. In the Anime the eyes are shown to simply be golden and shining red, and in the film they appear to have the appearance similar to that of a cats eye. This appearance of the Eye is only a representation and a means to show that a human has got the Shinigami eyes, it is otherwise impossible for other humans to see that they have Shinigami eyes, scientifically or not.Beyond Birthday (a former resident of Wammy's House, also known as either B or BB) was born with Shinigami Eyes. This allowed him to see the names and life spans of other humans, sending him slowly mad. It is still unknown to how it's possible for a human to be born with the eyes but a theory was briefly mentioned about a female Shinigami dropping an eye. Whether his is true or how BB came into possession of the eyes are unlikely to ever be revealed.
Shinigami eyes will only work if the human or Shinigami can see the face of the Human they intend to kill, if the face is only partially visible or obscured in any way the eyes will not work. Apparels such as glasses are also said to not have any effect on how well the eyes work so names will still be visible. It is also said that they can see the name and life span of humans if they are looking at a photo but not if they look at a drawing, no matter how accurate. If the person is deceased then the neither life span or name will show. This also occurs in video recordings.