You should be able to hear a perfect impact because the club hits the ball then the ground creating one sound, if you bottom out too early you will hear the impact of the club on the ground then the ball being hit
Place a cone behind you and practice your takeaway just trying to nick the top of the cone, if you hit the middle you are too far inside and if you take it back over the cone then your takeaway is going outside
It is much easier to aim at a target that is closer to you than one farther away, so pick an intermediate target a foot or two away from your ball and square your putter up to that spot and use the hole for distance control purposes