What is dharma and adharma is determined by scripture. Krishna says in the Gita (XVI. 23-24):
He who discards the injunctions of the scriptures and acts upon impulse of desire attains neither perfection nor happiness nor the Supreme Goal.
Therefore let the scriptures be your authority in determining what ought to be done and what ought not to be done. Having learnt the injunctions of the scriptures, you should do your work in the world.
But what is dharma for one person may be adharma for another person. For instance, it would be adharma for a sannyāsin to kill someone, but dharma for a soldier in war. Krishna says in the Gita (III. 35):
Better is one’s own dharma, though imperfectly performed, than the dharma of another well performed. Better is death is the doing of one’s own dharma; the dharma of another is fraught with peril.
But He does give some specific guides. Krishna says again in the Gita (XVI. 1-3):
The Lord said: Fearlessness, purity of heart, steadfastness in knowledge and yoga; charity, self-control, and sacrifice; study of the scriptures, austerity, and uprightness;
Non-violence, truth, and freedom from anger; renunciation, tranquility, and aversion to slander; compassion to beings and freedom from covetousness; gentleness, modesty, and absence of fickleness;
Courage, forgiveness, and foritude; purity, and freedom from malice and overweening pride—these belong to him who is born with divine treasures.
And further in Uddhava Gita (XII. 21) Krishna says:
Non-injury, truthfulness, freedom from theft, lust, anger and greed, and an effort to do what is agreeable and beneficial to all creatures—this is the common duty of all castes.
It is good to read all of chapters 3 and 16 of the Gita and chapters 5 and 12 of the Uddhava Gita for a good understanding of the subject.
Swami Nikhilananda in his writings on Hindu ethics (The Upanishads, Vol 2) says:
Besides the objective duties based on the castes and stages of life, there are laid down the common duties of men, the sadharanadharma, which are the foundation of the moral life. Manu, the lawgiver, enumerates these common duties as follows: steadfastness (dhairya), forgiveness (kshama), good conduct (dama), avoidance of theft (chauryabhava), control of the senses (indriyanigraha), wisdom (dhi), learning (vidya), truthfulness (satya) and absense of anger (akrodha)...the aim of Hindu ethics is to enable a man ultimately to conquer his lower self and attain freedom from passion, desire, and attachment.
All Hindu philosophers regardless of their conceptions of the supreme end of man, admit the empirical reality of the individual, endowed with volition, desire, will, conscience or consciousness of duty, emotion, etc. The goal of Hindu ethics is to train these faculties in such a way that they shall lead the individual to the realization of Moksha, or Liberation.
Therefore all the schools of philosophy have described the virtues and their opposites in detail. It is expected of the moral agent that he should follow the former and shun the latter. We propose to discuss the virtues and their opposites according to the classification of Nyaya and of Patanjali's system.
Vatsyayana, in his commentary on the Nyaya aphorisms, classifies will as impious (papatmika) and auspicious (subha). The impious will leads to unrighteousness (adharma), and the auspicious will, to righteousness (dharma). Righteousness, it is necessary to add, is conductive to the Highest Good, whereas unrighteousness produces evil. The purpose of ethics is to subdue the impious and to manifest the righteous will. Unrighteousness may take three forms, namely, physical, verbal, and mental, depending upon the condition of its functioning. Physical unrighteousness manifests itself as cruelty (himsa), theft (steya), and sexual perversion (pratisiddha maithuna); verbal unrighteousness, as falsehood (mithya), rudeness (katukti), insinuation (suchana), and gossip (asambaddha); mental unrighteousness, as ill-will (paradroha), covetousness (paradravyabhipsa), and irreverance (nastikya).
The practice of continence, highly extolled by all the philosophers and mystics of India, implies, besides the literal meaning of the vow, abstention from lewdness in thought, speech, and action through any of the sense-organs. Through the practice of this virtue, one develops the capacity for subtle spiritual perception.
So although there are actions which are specific to a person that lead to dharma, there are also actions which are for all people to follow that lead to dharma.