Are all employers legally insured?

Yes.

All employers who employ at least one employee are insured by one of two insurance companies: Generali Česká pojišťovna a.s. (for those insured as of 31 December 1992) or Kooperativa pojišťovna, a.s. (for all other employers). There is no need to find out if your employer has an insurance contract. None is concluded, as statutory insurance arises automatically when the first employment relationship is established with the employer.