Many employers avoid registering workers with social security (SGK). It is illegal but common. The law lets the worker recover those years.
Evidence
Pay slips, work correspondence, colleague witnesses, photos at the workplace, signed documents, even WhatsApp messages proving the relationship.
Witnesses
Colleagues who worked with you, customers, suppliers — anyone with direct knowledge.
Court
The labour court. The case is filed against both the employer and the social-security institution.
Outcome
Years are recorded retroactively, added to your social-security history, counted toward pension. You may also claim other unpaid entitlements.
Time limit
5 years for years up to 2007, with later refinements. Don't delay — proof gets harder over time.