According to current immigration law, employers have to regularly check the paperwork for a foreign worker, and must keep it on file for three years after hire or a year after termination, whichever period is longer. Employers can't, however, be held responsible for well-forged paperwork.
If you have any serious questions about immigration law, consult an attorney who specializes in the field. For professional reasons they have to keep themselves up-to-date on all the twists and turns on immigration law, and are far more likely to be able to help you than anyone else.
