The Perception Table

The basic most rule of perception is this: if someone's speed is greater then your perception you will not see them move unless you have a special ability that boosts your perception.

Perception Table:

+0: This is basic perception: no training, no special anything. This is the basis of what a human can do with absolutely no training at all.
+1 – 10: Trained Perception. This is slightly higher then normal perception. This is the sort of perception that comes with practice: tracking, avid hunters, bounty hunters, ect. At this point your perception is trained, but not perfected.
+11 – 15: Super Human Perception. All unpowered human movements do not escape the eye. Generally the peak of human ability. You notice movements in the dark and cannot be blinded by abilities that hinder vision.
+16 – 19: Supernatural Perception. You notice things at this point that others don't. This perception is beyond that of Super Human – even trained ones. You can now see in the dark very well; as if it were daylight outside. You can even sense some things that are nigh-invisible like spiderwebs due to increased resolving power of their eyes.
+20 - 30: Meta Human Perception. Besides the fastest of individuals, no species' movements escape your eye. With this level of Perception you have a higher visual acuity that normal lets you see everything in hi-definition.
+30 - 49: Demi God-like Perception. At this point the movements of the fastest individuals can be seen, but with difficulty. Any other species' movements will be perceived as moving in a slightly slower fashion; you can see bullets being shot from guns, and the flap of a humming bird's wings.
+50: God-like Perception. The pinnacle of sight, nothing escapes your notice, not even pure speeders (though still difficult to properly keep track of.)

Important things to remember
Higher leveled perks stack with lower ones.
Just because you may be able to see fast movements/fast attacks may not mean that you have the speed/reaction time to avoid/counter them. Pay attention to the stats!