How do you find someone's personal income?

If you mean how do you find out how much someone reported as taxable income on their return:

Barring someone that has been provided that information like a loan officer or such, you can't. It is against the law and agreements in most all information requests by outside parties, for anyone involved with a return or it's handling to disclose any information about it.

That even includes IRS employees, who CANNOT even access someones files without an authorization process and reason, and a record being made they did. If the information becomes known, the authorities would pursue how it was discovered.

I believe it falls not just under an invasion of privacy, but as none of your business!