When looking at an athletes snatch and clean technique, two of the first things I look at are: (1) weight distribution in the feet at the start of the lift and (2) foot movement between the pull and catch phase of the lift.
Weight Distribution in the Foot
Weight distribution at the start of the lift should be in the middle of the foot - near the balls of feet. There seems to be a common misconception that the weight should be in the heels during the snatch and clean. This is simply not true. I believe this comes from the idea that beginners are taught not to "come onto the toes too early". From my experience being heel heavy at the start leads to a very weak second pull and a less vertical bar path.
Most lifters should shuffle their feet out between the pull and the catch portions of the lifts. The start/pulling foot position should be comfortable and powerful. I tell lifters to do a vertical jump and see where their feet are before they jump. This is generally a good starting point to determine foot starting width. After the shuffle the lifter should end in their squat foot position. Usually a couple inches wider on each side then the pulling position. But this can vary for each lifter. I really encourage consistent foot position, the clean and snatch foot width in the catch should be identical to heavy front, back, and overhead squats.The outward foot-movement/shuffle should be quick and for most lifters will make noise. I often encourage my lifters to "let me hear your feet".These two quick corrections can go a long way in improving your Snatch and Clean, so remember (1) weight to the middle of the foot and (2) move feet out quick and aggressively!