Wage reporting when hours change weekly after 10 years as an electrician's helper

I work some days through a staffing agency while still certifying. I am in San Diego, in case local advice matters. The form asks for gross earnings, which I do not see until later. Who can I ask without risking my claim?

Thanks for writing this out. I went through something close to it 11 months ago, and it settled down once I kept a simple log of every call and form.