It's very simple - just flat out say, "I am not paid by Monsanto or by any organization that was contracted by Monsanto."
But you won't.
Check out this book for more information about why it's obvious that you're lying.
Also: >Would anything I say change the mind of someone who thinks I'm a shill?
Certainly! My goodness - all you have to do is provide reasonable proof that you're not.
Definitely true, but truly unrelated.