U.S. Department of Labor
Employee Benefits Security Administration
Public Disclosure Room
200 Constitution Avenue, NW, Suite N-1513
Washington, DC 20210
You can also call the IRS and request a heath plan audit.
Does the employer adminster the health plan? Maybe they hired someone else to do it (even though they are responsible).
You can also google or glassdoor.com your former company.
Problem is, if they were out of compliance, but then sent you the notice, they have remedied the defect. So the damages or your lawsuit might be minimal depending on the amount of days they did not send it. Tell me the dates of your situation, and we can calculate.