But it is not your responsibility, but that of the employer. You only need to claim, not prove that you made a certain amount. When the employer reports the income under your SS# or EIN to the IRS, then they look for matches on the appropriate returns.
Pretty sure you do not need to provide the W-9. I'll check Tuesday. On a 30 hour film score session Sunday and Monday.
And, I "AIN'T" giving the production company a W-9. They'll sure give me a 1099 at the end of the year, though.
For laughs, call one of your clients and ask them what they do with the W-9 you provide.
Be well,
Russ