If the condition of the investor and the company is satisfactory i.e. the income source of the investor is legitimate likewise the company is known for its halal business, then there is no need of further investigation. It is right to use the amount received from them without a doubt. In case the income of the investor or the company is doubtful then it is mandatory to check their source of income. And it is lawful for a Muslim to go with a government job in a non-Muslim country provided the job is of lawful nature though the Muslim is not paid the salary from halal money, as the validity and invalidity regarding doing job in government or non-Muslim private companies is based on action not on the (source of) salary. If the non-Muslim pays the salary of a Muslim from interest money then it is also lawful for Muslim to take the salary for his lawful job, as the non-Muslims are not bound to act upon the furu’ (derivative rulings of Islam). Yes, job in Muslim companies shall only be lawful when both the job and salary is halal and it is not lawful for a Muslim to take interest in any case whether the country is Islamic or non-Islamic.
Allah knows Best!
Darul Uloom Deoband