To know if you are practicing correctly: you are in a car and it is rush hour, the end of the day, the traffic is all stuck. You are sitting there for 5 minutes, 10 minutes, 15 minutes, 20 minutes, 25 minutes and you don't get upset. That means you are practicing correctly. That is the proof that you are practicing correctly. We don't judge practicing correctly on the pillow. How you act 24 hours a day is what's important.
Maybe you are eating with some friends and the dessert comes. There are four of you and four pieces of dessert come. Somebody grabs two real quick. How do you react? That will be another proof of whether you are practicing correctly. What I am talking about here is how peaceful, how compassionate, how loving you can be each moment of the day. How good is your reaction?
I personally know some meditators who can concentrate quite well. They can blank their mind, they can sit there like a rock, not move, and yet they get angry. Real quick.
We had one person at Wat Kow Tahm, 15 years meditating, about 40 years old. They got angry because our food was not hot. They got angry because they heard the motorboats 5 kilometers away. They even got angry because somebody sat where they liked to sit and they told the person to get out, even though it was a silent retreat. That person could concentrate and blank their mind but they were practicing incorrectly.
When you judge whether you are developing in this type of meditation, look at how you are reacting through out the day.
I will just add something about the word meditation. As far as I understand, in the Pali language there really is no such word. The common term in the Pali language that is often used in the scriptures is "Citta Bhavana". "Citta" means the heart-mind. "Bhavana" means development. So we have the words "mental development," that is what we mean by meditation in our practice. We are talking about a 24-hour developing of our minds.