Where can someone find effective feedback?

If you tell someone something that you think is important, or teach someone something, how do you know if there's any point?

If you ask someone what they think, the response that they give you will only be their reaction.
If you quiz them, you can at least find out what they learned.
If you observe their behaviour, you can actually infer whether anything has gotten through to them.
But waiting to see results of change takes a while.