US Citizen or Legal Resident verification form

Is anyone aware of a form a customer can sign certifying that they are either a US Citizen or a legal resident alien?

Is there a reason why we would not be permitted to use a certification form as I mentioned?

I remember reading earlier in the year that Bank of America would be certifying citizenship. I believe their existing customers had a problem with the questions. We however, would only use it to vet new customers. By policy with very few listed exceptions, we do not open accounts for NRAs. The problem lies with trying to determine if someone is a NRA. Of course signing a form doesn't make it the truth, but we have something to show that we made an effort.

Thanks in advance for any thoughts or answers.