Mahārāja Parīkṣit was such a pure devotee. He was wrongfully cursed by an inexperienced son of a brāhmaṇa, who was under the influence of Kali, and Mahārāja Parīkṣit took this to be sent by Nārāyaṇa. He knew that Nārāyaṇa (Lord Kṛṣṇa) had saved him when he was burned in the womb of his mother, and if he were to be killed by a snake bite, it would also take place by the will of the Lord. The devotee never goes against the will of the Lord; anything sent by God is a blessing for the devotee. Therefore Mahārāja Parīkṣit was neither afraid of nor bewildered by such things. That is the sign of a pure devotee of the Lord.
(Srimad Bhagavatam-----1:18:2-----purport).

Therefore, with the development of the bhakti cult, with the application of positive service to the positive form, one naturally becomes detached from inferior things, and he becomes attached to superior things. Similarly, the bhakti cult, being the supermost occupation of the living being, leads him out of material sense enjoyment. That is the sign of a pure devotee. He is not a fool, nor is he engaged in the inferior energies, nor does he have material values. This is not possible by dry reasoning. It actually happens by the grace of the Almighty.
(Srimad Bhagavatam-----1:2:7-----purport).

Brahmājī, being so questioned by Nāradajī, congratulated him, for it is usual for the devotees to become very enthusiastic whenever they are questioned concerning the Almighty Personality of Godhead. That is the sign of a pure devotee of the Lord. Such discourses on the transcendental activities of the Lord purify the atmosphere in which such discussions are held, and the devotees thus become enlivened while answering such questions.
(Srimad Bhagavatam-----2:5:9-----purport).

Prahlāda Mahārāja, the exemplary devotee of the Lord, had all the good qualities humanly possible. Although he was the emperor of this world, he was not profligate. Beginning from his childhood he was the reservoir of all good qualities. Without enumerating those qualities, it is said here summarily that he was endowed with all good qualities. That is the sign of a pure devotee. The most important characteristic of a pure devotee is that he is not lampaṭa, or licentious, and another quality is that he is always eager to mitigate the miseries of suffering humanity. The most obnoxious misery of a living entity is his forgetfulness of Kṛṣṇa. A pure devotee, therefore, always tries to evoke everyone's Kṛṣṇa consciousness. This is the panacea for all miseries.
(Srimad Bhagavatam-----2:14:49-----purport).

Śrīla Narottama dāsa Ṭhākura has sung:
anya devāśraya nāi, tomāre kahinu bhāi,
ei bhakti parama karaṇa
If one wants to become a pure devotee of the Supreme Lord, one should not hanker to take benedictions from the demigods. As stated in Bhagavad-gītā (7.20), kāmais tais tair hṛta jñānāḥ prapadyante 'nya-devatāḥ: those befooled by the illusion of the material energy worship gods other than the Supreme Personality of Godhead. Therefore, although Rantideva was personally able to see Lord Brahmā and Lord Śiva, he did not hanker to take material benefits from them. Rather, he fixed his mind upon Lord Vāsudeva and rendered devotional service unto Him. This is the sign of a pure devotee, whose heart is not adulterated by material desires.
(Srimad Bhagavatam-----9:21:16-----purport).

Āśā-bandhaḥ samutkaṇṭhā nāma-gāne sadā ruciḥ. This is the sign of pure, advanced devotee. He's never disappointed. Ahaitukī. There is no motive. "My Lord is there. My duty is to serve." That's all. "Whether I shall be benefited, whether my senses will be satisfied, gratified..." These are conditions. Unconditional. That is... Anyābhilāṣitā-śūnyam [Brs. 1.1.11]. Without any motive, without any condition, when we love Kṛṣṇa, that is first-class religion. Sa vai puṁsāṁ paro dharmaḥ. Para and apara. Apara means "inferior," and para means "superior." Just like there are two energies of Kṛṣṇa: para and apara, inferior and superior. So paro dharmaḥ means superior, the occupational duty in superior energy.
(Lecture on Srimad Bhagavatam----- 1:2:6).