Sight, touch, and hearing are all used intuitively in at least some way during combat, although not always to their full extent. With focus you can be sensitive to your opponent flexing his muscles to commit to an action. You can hear his friend sneak up behind you, or the bouncer coming to break up the fight, or the dog that's chained up a few feet behind you. You can see possible obstacles in your environment, as well as notice your opponent shifting his weight to prepare to move or strike. All in all, I wouldn't put too much faith in the eyes alone. After all, I'm not a chameleon, I can't see everywhere at once.
Other than that, the sense of smell can alert you to whether or not your opponent is inebriated and even what intoxicants he's using. He will smell different if he's stoned than if he's drunk. Even the fact that you can smell your opponent should be a warning of possible danger. In the normal course of the day, you walk past people hundreds of times without smelling them. If you suddenly find yourself noticing a person's scent, you should question why your body brought this to your attention right now. While it's true, you could have just come upon a particularly strong odered individual, you should still ask yourself why he smells. Does he bathe irregularly? Is it because he's lazy, or reclusive, or homeless? Is it his clothes? Is he sweating? Is it because he's hot, or drunk, or angry? Can you smell his breath? Is it because he's taking quick shallow breaths? Why is he standing so close to you that you can smell him? Does he smell sick? Could he be delusional with illness, or maddened by disease? Can you smell blood? Has he soiled himself in some way? All these things can effect the circumstances of combat.
The sense of taste is probably the least useful in combat. Other than as part of the sense of smell, it's difficult for me to think of ways in which it could help. I could see tasting smoke or sweat in the air, but that is part of smelling those things. Perhaps if you were grappling with an opponent you could taste his sweat and tell if he was drunk, but that doesn't sound very appealing at all. In fact, not tasting sweat seems to be a decent enough reason to avoid a fight to me.
As far as ways to train the senses, I've participated in many darkness drills. This is probably the easiest of all. Locking, trapping, grappling, striking, maneuvering, stances, forms, etc. can all be practiced with the eyes closed. By adding a partner, and in time, increasing the intensity, you become significantly less dependant on your eyes, and can see more with them open as a result.
I've trained in using the ears more effectively, but I've never tried kenpo with my ears plugged up. That could be interesting. Does anyone have any insight into whether or not it's harder to fight when you can't hear. I assume it would be harder to percieve attacks from your obscure zone.
When doing lock flow drills with the eyes closed you really have to focus on contouring and controlling space with your opponent. This helps to develop a sensitivity to your opponents movements. Doing the techniques with the eyes closed does the same thing, but has the added benefit of focusing on a specific dimension of a memorized pattern.
I'm not really sure what the best way to train the senses of taste and smell though. I try to train mine by eating only the best foods in the best restaurants, purely for training purposes.
And what of the sixth sense? Can you actually sense your opponents, "emotional intentions?" Some people believe any ESP phenomena is actually the result of the other five senses working in tandem. I suppose that would be like perceptual body fusion. Or is it all Spidey Senses Tingling?
-Rob
