Ask the employer for copies of the employment documents of all employees that will be working on your task. If they are reluctant to provide the copies, tell them that you'll find someone else. (That's usually a solid hint that they have reason to doubt the veracity of the papers.)
Next, check the SSNs with the Social Security Administration. If the number is invalid, stop right there and fire the contractor. If the number is valid, check the first three digits to see if they're from your immediate area, and ask the SSA to make sure if the account is getting FICA taxes from one region only. A non-English-speaking laborer with an SSN issued in Montana is a touch dubious.
Again I am not doubting you, but I find it hard to believe it is that easy or the SS administration will give you that information. You could be right and I will try if the situation ever occurs. What exactly do you mean by valid? Are you asking if, in fact, there is such a SS# or if it is a 30 year old male or a 70 year old female?
Of course, I will tell you most of these workers have more than one SS# - one of them is bound to seem good.
And I am sure you do this every time you go to McDonalds or Burger King to get some fries and/or a coke. Sure you do.
Just another principled republican.