Types of Meditation
If you are meditating on a form, a sound or an object then the only real way to know you are meditating correctly is to behold that form, the sound or the object in front of your inner eye. If it looses its sharpness you know you are not meditating correctly and your mind has gone off to some other train of thoughts. If you are meditating on thoughtlessness or formless, it is one of the highest forms of meditation because when you are meditating on the formless your mind is looking at itself. It is the ultimate state to be in. It’s like the waves merging in the ocean and you become the ocean. There is nothing left. You loose that individual I-ness that I exist. The individual existence forms the part of the ego and you lose it completely.
Once somebody asked Ramakrishna Paramhansa that I only believe in meditating on the formless and that is where I want to start. Ramakrishna replied to him that when an archer starts to learn archery he begins by shooting at a still object and once he is proficient in shooting at that then he starts aiming at moving objects.
Listen to this discourse by Om Swami on the types of meditation and the barriers to meditation.