Who is a pure devotee?

anyābhilāṣitā-śūnyaṁ

jñāna-karmādy-anāvṛtam

ānukūlyena kṛṣṇānu-

śīlanaṁ bhaktir uttamā

When first-class devotional service develops, one must be devoid of all material desires, knowledge obtained by monistic philosophy, and fruitive action. The devotee must constantly serve Kṛṣṇa favorably, as Kṛṣṇa desires - CC Madhya Lila 19.167

The basic criterion to fulfill the above verse is that a devotee must know what Krishna wants Him to do. Krishna's desires can be boiled down to three core points (vide Srila Prabhupada purport);

1. One must take shelter of a spiritual master who comes in the disciplic succession from Kṛṣṇa - ādau gurv-āśrayam
2. The first business of the serious and sincere disciple is to satisfy his spiritual master. The spiritual master's only business is to spread Kṛṣṇa consciousness.-yasya prasādād bhagavat-prasādaḥ.
3. The serious devotee will only be interested in satisfying Krishna. Krishna wants everyone to be His devotee and not the devotee of a demigod - Man-manā bhava mad-bhakto mad-yājī māṁ namaskuru

These are the desires of the Supreme Lord, and one who fulfills His desires favorably is actually a pure devotee.

Hare Krishna