Usually in the same way it's done everywhere else: name, date of birth, registered/current address, sometimes an additional pin/security question/etc, depending on who you're calling.
If it's official business you might be required to send an actual letter, though I doubt they ever check your signature unless you're suing them.
If it's online and state business (portal for unemployment stuff, state employee pension details, etc.), you usually have to provide your name and address first, which they then check against your registered address. They'll then send you a letter with a one-time password you can use to register your account.
Edit: Modern E-Business companies often require you to "verify" your identify by ways of Postident (you present your ID to a post office) or IDnow (you present your ID to a random guy via webcam who asks you to move it around so he can see all holograms, data, etc.) and can compare it to your picture in the webcam.
This is considered to be enough for financial transactions according to our current money laundering laws, so it's about the most though version you can go through.