Under most circumstances, private employers cannot ask you about any convictions dismissed under Penal Code §1203.4. That's good, because for the most part if you're applying for a job in the private sector, you do not have to disclose a conviction if it was dismissed or expunged.
Unfortunately, when applying for government employment or a government you must disclose the fact that you had convictions, and that they were dismissed. For the most part, in California, even though you're required to disclose the conviction government employers and licensing agencies will treat you the same as if you had never been convicted of any crime (except for employment in law enforcement).
I must respectfully disagree with Ms. Guerra. Most California State licensing agencies are extremely concerned about license applicants' prior criminal convictions and will make getting the license (or keeping it if already issued) a challenging legal fight. They are particularly resistant to issuing an unrestricted license if the prior criminal convictions are theft-related or drug-related.
Licensing law is a distinct legal specialty and only long experience with the individual licensing Boards and Commissions enables an attorney to accurately predict in a given case which prior convictions are likely to be an impediment to occupational/professional licensing in California.