I always find that I use the golf swing as an example of what an instructor does for you.
I know that if I'm always slicing the ball, then there's a variety of things that could be messed up. One of them being that I'm not taking the club straight away from the ball. But I'm only good enough to tell when swinging a club that I'm 15 degrees inside or 15 degrees outside. Once it gets down to a 10 degree or less difference, all the swings feel the same to me, but my ball still slices. A good instructor will stand behind you and repeatedly tell you until you learn what it feels like. And give you some drills to fix things on your own.
One drill I picked up in snowboarding lessons was to hop a little in a traverse. If the board leaves the snow evenly, then you're pretty centered. If it rises tip first then tail, you're in the back seat. So now, whenever my turns feel a little sloppy or I find I'm a little sketchy on the ice, I'll take a couple of turns, hop a couple of times and just check that I'm not creeping a little into the backseat. Then I can adjust right then and there. That little self-test was something that I never would have learned from a video.