First of all, it is recommended that you wait until you hire an individual before asking him/her for papers to verify his/her identity and work authorization. However, if you ask for papers ahead of time only from people who appear to be "foreign," you are discriminating on basis of national origin. You must treat all applicants equally, and, when you review their papers, you cannot insist on seeing particular documents if they have already shown you valid documents. Otherwise, you are engaging in document abuse.