What is the correct definition of Zulm? How can I be sure that what I am doing to others is not Zulm?
Zulm (lit: injustice, wrongfully depriving someone of his legal and moral rights), from a moral perspective, means to wrongfully take something for oneself or grant it to another person, and to, thereby, deprive the person, who actually had a rightful claim on that thing.
As for the second part of your question, I would suggest that you should carefully and objectively analyze your dealings and transactions with others in the light of the foregoing explanation of ‘Zulm’ and then try to determine whether or not any of your actions entail an element of Zulm. You may even take the opinion of your associates and colleagues in this regard. You must also remember that an individual shall be held accountable and punished for only those acts of injustice, which he had knowingly committed. God, with His all encompassing mercy, has promised to pardon us for our honest mistakes and errors.