It is believed that one needs to be initiated by a bona-fide spiritual master in the bona-fide disciplic succession, otherwise the mantra he learnt from any other sources is useless. The proper way to learn scriptures is that one should approach a guru in the bona-fide sampradaya. Now, my question is that how can I conclude that which disciplic succession is bona fide and which is not? Do scriptures say anything about this?
Any valid sect (Sampradaya) must start with God. And then it comes to us through the 3 lines (Pangktis) of Gurus viz- Divyauga (Divine Gurus), Siddhaugha (Celestial Gurus) and Manavaugha (Human Gurus).
For Kaulism (a branch of Shaktism) these 3 lines of Gurus are as follows:
Divyaughe ChAdinAthashcha Tacchshaktischa sadAshivaha|
Tathpatni Cheswarastasya BhAryA Rudrascha Tadvaduhu||
Vishnushcha TathpriyA BrahmA TathkanthA DwAdaseritAha|
The Divyauga Guru Pangti consists of 12 Gurus viz - Adinatha, Sadashiva, Iswara, Rudra, Vishnu and Brahma along with their respective shaktis or consorts.
Siddhaughe Sanakaishchaba Sanandashcha SanaAtanah|
SanatkumArascha SanathsujAtascha Rhibhukshajah||
DattAtreyo Raibtako VAmadevastathaha Parma|
Tato VyAsah Sukaishchava Ekadasha SamiritA||
The Celestial Gurus are 11 in number and they are - Sanaka, Sananda, Sanatana, Sanathkumara, Sanath-Sujath, Rhibhukshaja, Dattatreya, Raivataka, Vamadeva & Vyasa.
MAnvaughe Nrisimhashcha Mahesho BhAskara TathA|
Mahendro MAdhavo Vishnu Shadethe Cha PrakirtithAha||
The human line of Gurus consists of six Gurus namely - Nrisimha, Mahesha, Bhaskara, Mahendra, Madhava & Vishnu.
KulArnava Tantram, Chapter 6, Verses 64-67.
So, the Sampradaya started from Adi Natha (or Shiva) and the knowledge (Mantras etc) passed on to other deities and then to the celestial beings and finally to the chosen human beings. In this manner finally we receive Mantras etc from that particular Sampradaya Gurus.
For the Natha Sampradaya one such Parampara is:
Adinatha → Atrinath → Dattatreya → Janardana → Ekanatha → Nityananda → Krishnananda → Vishibananda → Muhranatha → Ranganatha → Gopalanatha → Balabhadranatha → Dilvarnatha → Sundarnatha → Brahmanatha → Gambhirnatha → Digvijayanatha → Navaminatha → Kamalnatha → Mithileshnatha → Matsyendranath
,which again started from Adi Natha (Shiva).
For another Sampradaya, called Trika Kaula (popularly known as Kashmiri Shaivism), the various lines of the Gurus are given in this answer.
So, when you approach any Sampradaya for Diksha you should be able to verify from them whether that particular Sampradaya started from God or not and also how it propagated. Only if it started from God it is a valid sect, otherwise not.
All bonafide Sampradyas have such lists of Guru Paramparas which usually will have the 3 lines (Pangktis) I have mentioned.
There is no 'pope' in the Vedic tradition, no ultimate judge as to who is 'bona fide' and who is not. Many teachers, including those who are misguided, claim descent from various lineages of teachers, so claimants of a certain lineage is not a means to judge a teacher or a sampradaya. The exact lineage of all sampradayas is lost in the fog of time. The Mundaka Upanishad I.ii.12 says (Swami Nikhilananda translator):
Let a brahmin, after having examined the these worlds that are gained by works, acquire freedom from desires; nothing that is eternal can be produced by what is not eternal. In order that he may understand that Eternal, let him, fuel in hand, approach a guru who is well versed in the Vedas and always devoted to Brahman.
and Sri Shankaracharya says in his work, Upadesa Sahasri (A Thousand Teachings) I.1.6, Swami Jagadananda translator:
The teacher is one who is endowed with the power of furnishing arguments pro and con, of understanding questions (put by the disciple) and remembering (so as to answer them) them, who possesses tranquility, self-control, compassion and a desire to help others, who is versed (through instructions handed down to him) in the scriptures and unattached to enjoyments of both seen (i.e., both here and hereafter) and unseen, who has renounced the means to all all kinds of actions, who is a knower of Brahman and established in It, is never a transgressor of the rules of conduct, and who is devoid of shortcomings such as ostentation, pride, deceit, cunning, jugglery, jealousy, falsehood, egotism and attachment. He has the sole aim of helping others and a desire to impart the knowledge of Brahman only. He should first of all teach the Sruti texts establishing the oneness of the Self with Brahman such as "My child, in the beginning it (the universe) was Existence only, one alone without a second" (Chandogya Upanishad 6.2.1), "Where one sees nothing else" (Chandogya Upanishad 7.24.1), "All this is but the Self" (Chandogya Upanishad 7.25.2), "In the beginning all this was but the one Self" (Aitareya Upanishad 1.1.1), and "All this is verily Brahman." (Chandogya Upanishad 3.14.1).