What proves the service
Under article 125 of the Labour Code, the work book, on paper or in electronic form, is the main document that proves the length of service. Further evidence: copies of employment contracts and orders; wage certificates; electronic records of social insurance contributions.
The electronic system
The data about your work activity are kept in an electronic system. Check the records from time to time — a period that was not entered becomes a problem later.
If a document is lost
Ask the archive of the organisation for a certificate. If the organisation was liquidated, apply to the state archive. If this is not possible, a court can establish the length of service. Under article 295 of the Civil Procedure Code, the court establishes a fact only when the document cannot be obtained in another way or restored.
Advice
Every time you leave a job, take a copy of the order and a certificate. Under the Labour Code, the copy of the dismissal order is given on the day of the dismissal (article 171). The employer gives other documents related to the work free of charge, on a written application, within three working days (article 117). Collecting them later is much harder.