How can one distinguish between the lawful (halal) and the unlawful (haram) while avoiding doubtful matters (shubuhat) in work and life?

One cannot avoid the forbidden (haram) and doubtful (shubuhat) matters except by knowing the ruling of Allah Almighty. For knowledge of the Shar'i rulings is the foundation of goodness, and for this reason, seeking knowledge is an obligation upon every Muslim. It is not permissible for a mukallaf (accountable person) to perform an action except after knowing Allah's ruling on it, or by asking scholars. And whoever wishes to know the lawful from the unlawful should study the Book of Allah and the Sunnah of His Messenger, and the books of the people of knowledge, or ask them.