Metsuke refers to using your eyes.
"Me" means eye and "Tsuke" means to attach or adhere. Virtually directly translated this means that you attach your eye to your opponent. Not letting him or her out of your sight. To look at your opponent.
There are two methods of Metsuke.
Ken No Me and Kan No Me.
Ken no me is the method a beginner uses to look at his opponent. It has a basic and and advanced level.
Basic Ken No Me
Hence, it is said to always look in the eyes of the opponent with your eyes. This is Metsuke on a lower level - a basic level. The eyes are often referred to as the windows to the soul, which means that intention can be seen. The eyes of your opponent will tell you what he is going to do.
Advanced Ken No Me
Depending on the level of your opponent it might be smarter not to look him in the eye as he will be able to read you as well. Instead, look through your opponent, about one or two meters behind him, this is ‘Toyama No Metsuke’. See the distant mountain very sharp, but all that is close blurry. Never look at your target or just observe his hands or feet. If you look at his feet, you can’t see his hands and vice versa. So, you should see him as a whole. Toyama No Metsuke is an advanced form of Ken No Me, but it can still be an act of merely seeing with your eyes.
Kan No Me is different.
Seeing the opponent as a whole should be like feeling the opponent. Ishikawa sensei said “I could smell it” referring to what the opponent was going to do. This is a higher level of feeling and sensing the opponent. This is called Kan No Me.
Kan is translated at perception, intuition or the sixth sense. It is not very scientific, so we have to view this historically.
When there is a lack of scientific evidence humans tend to fills the gaps with ideas, assumptions - such as the sixth sense. There is no such thing.
But Kan No Me is not Ken No Me. So what is it all about?
This is what I can say from my personal experience. And from what I was taught by my sensei.
Feeling is everything. Scientific or not.
Experienced budoka know the difference between Ken and Kan No Me. Not being able to explain it scientifically does not make it less true.
Just bear in mind that an explanation such as "sixth sense" is figuratively speaking. It is the way of using words for knowlegde that is hard to put in the right words.