Each court and county is different. Generally, if you are not able to hire a private attorney the judge will ask you if you intend to hire an attorney, or if you are requesting the Public Defender be appointed to represent you. The court will make an inquiry as to your financial resources,. Often you will be directed to complete a financial evaluation form - disclosing your income and assets. Based on your financial evaluation it will be determined whether a public defender will be appointed. Ask the court and public defender's office in the court your case is pending out of for further, more specific, details.
I know of a situation here in Los Angeles County where a Public Defender was appointed for someone who jointly with his spouse had an income of over $200,000 a year. Apparently, the accused made a pretty good case that even with the income, given 4 kids in college, etc., he did not have enough money for a private lawyer. My point here is that requirements can be flexible and you are always free to attempt to make your case to the Public Defender's Office, or the judge if you need to, that you qualify for help.
That said, you are likely to get a bill from the Public Defender's office at the end of your case anyway, especially if they know you have some income coming in. Accordingly, you might consider retaining a private lawyer from the beginning. While certainly there are some terrific public defenders out there, a good private lawyer will most likely devote a lot more time to you case.
Jacek W. Lentz, Esq.