What are my responsibilities if I suspect too many dependents being claimed

Get age, relationship, how many months they lived with taxpayer, salary of each dependent, college status. Make sure your contract with the client says you are responsible for wrong information that they give you, or information they fail to give you.

For dependent relatives there is an income test -- their income has to be ridiculously low. I'm not sure that they must have lived with the taxpayer for more than 6 months of the year, but look it up.

Sounds like it's the Additional Child Tax Credit that you are primarily concerned about. The previous response gave good advice, but new this year form

8812 (Additional Child Tax Credit) explicitly asks for additional information about child dependents with ITINs.

However, those questions don't thoroughly address whether the child is in fact a qualified child to the taxpayer, which is the key factor. You have to be comfortable what the taxpayer is telling you or you're better off passing on them. You'll sleep better.

Do you mean _not_ responsible? If not, why do you want this in the contract?

Thanks that was a typo on my part. "not responsible" is what it should be.