There is an interesting verse in the Bhagwad geeta which is as follows:
sthita-prajñasya kā bhāṣā
sthita-dhīḥ kiṁ prabhāṣeta
kim āsīta vrajeta kim
In this verse Arjun asks Lord Krishna that how does a person who has a stable mind speak, sit and walk?
He tells that as a rich or a poor or a diseased person have certain symptoms through which he can be recognised as a rich, poor or a diseased person respectively, a man who is in transcendental consciousness can also be recognised.
There is a verse in which Lord Krishna further explains his qualities. The verse is given below:
prajahāti yadā kāmān
sarvān pārtha mano-gatān
ātmany evātmanā tuṣṭaḥ
In this verse he explains that when a man is relieved from all the sense desires and when his mind finds total satisfaction in itself, then such a man is said to have achieved transcendental consciousness.
He tells that it is easy for relieving one self from material desires by focussing on Krishna consciousness without any external efforts. When the soul is enjoys in itself thinking it to be the eternal servitor to the God, then such transcendence is said to be achieved.
He further explains it in the following verse:
sthita-dhīr munir ucyate
He explains that he who is neither elated when there is happiness nor is distressed when there is sorrow, he who is neither attached nor detached, he who neither has fear nor anger is the person with a stable mind. He who is not agitated with all of these things is said to have attained a pure transcendental stage.
One more verse explains it further:
tat tat prāpya śubhāśubham
nābhinandati na dveṣṭi
tasya prajñā pratiṣṭhitā
In this verse he explains that there are many dualities in this material world, like:
and many more and all these things either produce attachment or detachment in one or the other way. He explains that he who is not disturbed by all these, he who neither suffers from attraction nor aversion. Such a person who is not agitated by such dualities is the person with a stable mind. He never gets angry when his attempts are unsuccessful.
Sense attachments are very strong. Even if a person doesn’t want to be controlled by his senses, it is very unlikely that he will be able to do so but a person who doesn’t work for the gratification of his senses and is free from all the dualities and attachments and aversions that arise due to his senses then such a person’s mind is fully fixed on Krishna consciousness and he is always steady in his determination.