How would a customer verify that? They are supposed to also monitor the worksite? And be knowledgeable about consistent work and the type of PPE it requires?
It's not really that hard to understand PPE requirements.
Is it dusty? Wear a mask.
Is it loud? Hearing protection.
And everyone should wear shoes with steel caps.
If you see someone with a bandana over their mouth in sandals you know they are unsafe. They know it too. This is not something that is difficult to understand.
And then what? The customer takes pictures and reports them to the government? Fires them? What if there is a dispute, the laborer takes the customer to court for false claims?
Who polices the customer? The logistics of making everyone a cop make no sense to me.
If you see your contractor not using PPE, you just tell them to put it on. Just like you'd tell him to do it properly if you saw him cutting corners somewhere else.
Contractors want to get paid, so they generally tend to do what the person paying the bill asks them to do.
