How do we judge those around us regarding the lawfulness of their earnings and whether they are suitable for marriage (مصاهرة) or partnership in business, especially if they exploit their positions for personal gain and accept gifts due to their influence?

The default presumption for a Muslim is one of concealment and the integrity of wealth, unless proven otherwise. This is based on the Prophet's (peace be upon him) saying: "Beware of suspicion, for suspicion is the falsest of speech." If it is proven that the earnings are illicit, such as exploiting positions or accepting gifts that are considered ghulul (illicit gain), then it is better not to associate with them, based on the Prophet's (peace be upon him) saying: "Leave that which makes you doubt for that which does not make you doubt."

Marrying off a daughter to a profligate person is considered severing the ties of kinship with the daughters and causing them to be lost, as Allah Almighty says: "Protect yourselves and your families from a Fire." As for your marrying their daughters, its ruling varies depending on the circumstances. The principle is to seek righteous in-laws. The scholars have explicitly stated the dislike of marrying the daughter of a profligate person, based on the Prophet's (peace be upon him) saying: "Choose carefully for your progeny, and marry those who are compatible, and marry into their families."