A professional nail technician should demonstrate to you during services that they are providing client protection to you. They should explain what steps they take to sanitize and disinfect their implements, and what type of disinfecting products they use.
Any file that cannot be disinfected must be disposed of after use on each client. If you see a technician pick up a used or dirty file or other implement, including drill bits, DO NOT LET THEM USE IT ON YOU. Make them use a new file or show you that it has been properly disinfected before they use it on you.
In many US states you can check to see if the salon has violations. See the link in the Other Reading Material section to your left: File a Complaint. Here you can find links to the agency that governs salons in your state.