to ask some one about his monthly income
It is considered impolite to directly ask someone about their monthly income. However, if you need to discuss financial matters or inquire about their job or career, you can ask indirectly by saying something like:
1. "What field do you work in?"
2. "What is your profession?"
3. "How is your job going?"
4. "Do you enjoy your work?"
5. "Can you tell me about your career goals?"
6. "What do you do for a living?"
7. "Are you satisfied with your current financial situation?"
Remember, it is always best to approach these subjects cautiously and respectfully.