One should perceive the Lord in a gross conception of the universal form. That is why Krishna in the Bhagavad gita also describes, upon being questioned by Arjuna. Kesu kesu ca bhavesu cintyo ‘si bhagavan maya - in what material form can I think of You? So while chanting and hearing, the holy name of the Supreme Lord, one should perceive the presence of the Lord in His material manifestation.
We perceive Krishna through this material manifestation first, then gradually by spiritualizing the mind and senses, one is able to concentrate the mind upon Lord Vishnu who is present as a super soul in every heart. So the form of Vishnu as paramatma is for the purpose of realising the presence of the Supreme Lord within oneself and within the heart of every living entity. Now this realization helps the devotee to actually focus his concentration on the form of the Lord but not material form. In the Bhagavad gita, after the description of the universal form and the manifestation of the gross forms, there is a description of the Lord, how not only in His gross manifestation but also as the Controller - Ishvara. He is actually the ultimate Controller of every living being, or everything - He gives knowledge, He is the sanctioner, He is everything. In this manner, the devotee becomes conscious of the presence of the Lord in every step of his life. With he chants realising this, it results in a change in heart, change of consciousness characterized by ecstasy, when tears fill the eyes and hairs stand on end.
There can be pseudo devotees sometimes. They are somehow making a show of devotion, trying to cheat innocent people or trying to get some cheap adoration. So how to detect whether somebody is actually advanced or is imitating? The scriptures describe that not only externally there are symptoms of ecstasy in an advanced devotee but there are other symptoms. These symptoms are described by Rupa Goswami in Bhakti-rasamrita-sindhu - ksantih - the first symptom of a person who is really advanced is that he is free from all material desires. The pseudo devotees are full of material desires. There are other symptoms also - avyartha kalatvam - an advanced devotee is always interested in spending every moment in the service of the Lord. They have no attraction for any material enjoyment - viraktih - detachment. Then fourth one is mana sunyata - completely prideless. They are not interested in receiving honour from others. The fifth one is asa bandah - they are always hopeful for the mercy of the Supreme Lord. Krishna is the maintainer, Krishna will provide for me, Krishna is my well wisher - asa bandhana. Then samuthkantah - which means he is always eager to glorify the Lord. All these symptoms are never found in a pseudo devotee. Nama gana sada ruci - and he has taste for always chanting and hearing the holy name of the Supreme Lord. Asaktih tad guna akhyane - and they want to always glorify the Supreme Lord. Just like Prabhupada. Wherever he went, whatever conversation he was having, he was always glorifying Krishna. Then last one - pritis tad vasati sthale - always attracted to living in the land of the Lord, just like living in a temple, living in a holy place connected with the pastimes of the Lord. So these different characteristics distinctly separate a real devotee from a pseudo devotee.
So Prabhupada says that one begins devotional service by hearing and chanting. But in a neophyte state, one should not imitate the advanced devotees by making a false show of devotion. One has to wait for the heart to actually change. One more characteristic of an advanced devotee is that - just like Lord Chaitanya, when he met Ramanandaraya, he actually developed all the symptoms of ecstasy, but because there were some non devotee brahmanas accompanying Ramanandaraya, Chaitanya Mahaprabhu suppressed these symptoms. Simply by seeing some external symptoms we cannot recognize a pure devotee who has come to the stage of bhava. If there is no change of heart manifesting these external symptoms of ecstasy in spite of continuously chanting and hearing, that is only due to offences committed while chanting the holy names of the Lord. Even though we are engaged in chanting and hearing the holy name of the Supreme Lord, we should avoid the ten offences against the holy name. Therefore Prabhupada has made this system so that we remember to avoid these offences while executing devotional service.
Through Guru Prabhupada says that if somebody is serious about really making advancement, then one has to strictly follow the instructions of the revealed scriptures as directed by a realised devotee. So somebody may think what is the need for a spiritual master? The reason is that certainly chanting will benefit a person but he cannot make advancement on the path of devotional service without the expert guidance of a realised devotee. At some stage, he will realise the necessity of getting the specific direction of the realised devotee. Therefore Rupa Goswami advises in the Bhakti-rasamrita-sindhu - adau guru asrayam - sixty four principles on vidhi bhakti or vaidhi bhakti - following the rules and regulations for a beginner. The first thing is to approach a bonafide spiritual master and take initiation from him, instructions from him, right in the beginning so that you are not misled and you are able to make proper progress very quickly.