In other practices - like karma-yoga, jnana-yoga and dhyana-yoga - one may not be confident about his progress, but in bhakti-yoga one can become directly aware of his progress in spiritual life
SB Canto 4
When a devotee takes shelter at the lotus feet of the Supreme Personality of Godhead, he is completely cleansed of all misunderstanding or mental speculation, and he manifests renunciation. This is possible only when one is strengthened by practicing bhakti-yoga. Once having taken shelter at the root of the lotus feet of the Lord, a devotee never comes back to this material existence, which is full of the threefold miseries.
As stated by Lord Caitanya Mahāprabhu in His Śikṣāṣṭaka instructions, by the chanting of the holy name of the Lord—Hare Kṛṣṇa, Hare Kṛṣṇa, Kṛṣṇa Kṛṣṇa, Hare Hare/ Hare Rāma, Hare Rāma, Rāma Rāma, Hare Hare—or by the process of hearing and chanting of the glories of the Lord, one's mind is gradually cleansed of all dirt. Due to our material association since time immemorial, we have accumulated heaps of dirty things in our minds. The total effect of this takes place when a living entity identifies himself with his body and is thus entrapped by the stringent laws of material nature and put into the cycle of repeated birth and death under the false impression of bodily identification. When one is strengthened by practicing bhakti-yoga, his mind is cleansed of this misunderstanding, and he is no longer interested in material existence or in sense gratification.
Bhakti, or devotional service, is characterized by vairāgya and jñāna. Jñāna refers to understanding that one is not his body, and vairāgya means disinterest in sense gratification. These two primary principles of separation from material bondage can be realized on the strength of bhakti-yoga. Thus when a devotee is fixed in the loving service of the lotus feet of the Lord, he will never come back to this material existence after quitting his body, as confirmed in Bhagavad-gītā by the Lord (tyaktvā dehaṁ punar janma naiti mām eti so 'rjuna (BG 4.9)).
In this verse the word vijñāna is specifically important. Jñāna, the knowledge of spiritual identity that one attains when he does not consider himself to be the body, is explained in Bhagavad-gītā as brahma-bhūta (SB 4.30.20), the revival of spiritual realization. In the conditioned state of material existence one cannot be spiritually realized because he identifies himself materially. The understanding of the distinction between material existence and spiritual existence is called jñāna. After coming to the platform of jñāna, or the brahma-bhūta state, one ultimately comes to devotional service, in which he completely understands his own position and the position of the Supreme Personality of Godhead. This understanding is explained here as vijñāna-viśeṣa. The Lord says, therefore, that knowledge of Him is vijñāna, science. In other words, when one is strengthened by scientific knowledge of the Supreme Personality of Godhead, his position of liberation is guaranteed. In Bhagavad-gītā (9.2), the science of devotional service is described as pratyakṣāvagamaṁ dharmyam, direct understanding of the principles of religion by realization.
By practicing bhakti-yoga, one can directly perceive his advancement in spiritual life. In other practices—like karma-yoga, jñāna-yoga and dhyāna-yoga—one may not be confident about his progress, but in bhakti-yoga one can become directly aware of his progress in spiritual life, just as a person who eats can understand that his hunger is satisfied. Our false appetite for enjoyment and lordship of the material world is due to a prominence of passion and ignorance. By bhakti-yoga these two qualities are diminished, and one becomes situated in the mode of goodness. Gradually surpassing the mode of goodness, one is situated in pure goodness, which is not contaminated by the material qualities. When thus situated, a devotee no longer has any doubts; he knows that he will not come back to this material world.