My son says he works at Amazon in Dupont Wash but you can't contact them. I want to know if he's telling the truth. How do I find out?

Ask your son to show you a pay stub, or employee ID (they would use them there)
Also many jobs there are filled by employment companies who provide temp labor.

But they are hiring, if you don't have a criminal record, have proper ID and can do manual labor, (and high school degree) you can walk in, interview and get job the same day. They are short of help.

You don't have the legal right to know, if he is 18+.
If he lives with you or if you support him in any way financially, you of course have more rights. You ask for proof and if he doesn't provide it, your 'rights' are basically to throw him out or take away support.

Having said all that, I believe him. The amazon warehouses are bigger than airplane hangers, full of miles of shelves of products, and employees can't use phones. Why do you have a 'need' for proof?
Sometimes parents have to let their children struggle with being young adults on their own for several years. They'll change as they mature. Let them.

I think the bigger issue is that you don't trust your son. Is there a reason for this?

How old is he? Is he living with you? Are you supporting him? Why do you feel you're entitled to proof that he's working?