How can someone know what he/she doing is correct or wrong?

There is a saying in Bengali - phalena parichiyate - something is judged by its results, or like Jesus says - a tree is judged by its fruits.

In Bhagavad Gita, Krishna explains how nature operates according to the three modes of material nature - goodness, passion, and ignorance. The mode of passion creates, the mode of ignorance destroys, and the mode of goodness preserves and maintains.

That means, the more a person is under the influence of goodness, the more right he is, and the more one is influenced by ignorance, the more wrong one is.

Again, this can judged by the results of one’s actions. Actions performed in the mode of goodness are long term beneficial for oneself and others, and actions performed in the mode of ignorance, are destructive to oneself and others.

Actions performed in the mode of passion may give some short-term benefit, but will often degrade into ignorance, and thus whatever benefit it generated will be finished. For instance, killing animals for food, may be good for some short term profit, but in the long run it will bring about suffering for oneself.

Giving food to a hungry person is an action in the mode of passion. Soon the person will be hungry again, so the benefit is short term. To give the person some land and teach him to grow his own food is an action in the mode of goodness, because it is long term beneficial.

In the Vaishnava process of knowledge, there are three ways of having one’s activities verified - guru, sadhu, and sastra - the spiritual master, the tradition, and the scripture. These three must all correspond to assure the right outcome of one’s thinking or doing. A guru, or a holy man cannot direct you in a way not sanctioned by the scripture.

So, for knowing the right directions in life, to know the principles by which one should live, one must approach a teacher - a spiritual master. A spiritual master is a life-guide who connects us to God.

Srila Prabhupada had a very simple rule of life - if you doubt, don’t - meaning, if one is not sure about the outcome of a certain course of action, one should desist from doing it.

Krishna says:

Just try to learn the truth by approaching a spiritual master. Inquire from him submissively and render service unto him. The self-realized souls can impart knowledge unto you because they have seen the truth.

Having obtained real knowledge from a self-realized soul, you will never fall again into such illusion, for by this knowledge you will see that all living beings are but part of the Supreme, or, in other words, that they are Mine.

Even if you are considered to be the most sinful of all sinners, when you are situated in the boat of transcendental knowledge you will be able to cross over the ocean of miseries.

As a blazing fire turns firewood to ashes, O Arjuna, so does the fire of knowledge burn to ashes all reactions to material activities.

In this world, there is nothing so sublime and pure as transcendental knowledge. Such knowledge is the mature fruit of all mysticism. And one who has become accomplished in the practice of devotional service enjoys this knowledge within himself in due course of time.

A faithful man who is dedicated to transcendental knowledge and who subdues his senses is eligible to achieve such knowledge, and having achieved it he quickly attains the supreme spiritual peace.

But ignorant and faithless persons who doubt the revealed scriptures do not attain God consciousness; they fall down. For the doubting soul there is happiness neither in this world nor in the next.

—Bhagavad-gita, 4.34-40