How to prove that you've worked as a freelancer

If one freelanced in a country outside Germany, or even here for that matter, how does one go about "proving" employment if need be? I have invoices from when I did this abroad, but I can't think of any official documentation or certificated that I could provide, unlike if I'd worked for a company as an employee. Do employers here tend to be rigid when it comes to this kind of stuff? Or is it case by case? Thanks for any info.

I don't quite understand what you mean, do you need to show someone (i.e. hire purchase) proof that you're employed? I freelance here and abroad and have never had to show anyone that I am in work but I have been asked by companies to prove that I am not a money grabbing b'stard who doesn't pay any taxes. That is the reason I have a MwSt number along with some other official loking bits and pieces from the Finanzamt. As for working in other countries I just show my residents permit to say I pay tax here and not there (although for Holland I had to sign a declaration)

If you want to register as a freelancer in Germany you need to prove that you will have contracts to fulfil from a number of potential customers. You can do this with a comprehensive business plan (or normally) that other people or companies are willing to use your services. A letter from 2 or 3 will suffice. They have little or no interest in your previous customers (unless you will continue to service them)

I don't entirely understand your question. Do you want to show a prospective employer or client what work you have been doing, and prove where you have been working ?

If so, then write a detailed account of what you did as a freelancer, detailing projects etc. Provide details of your clients. Be prepared to answer a lot of questions and use invoices as evidence you have been doing paid work.

If you have been paying taxes then somewhere on the tax statement your status as a freelancer should be shown.

Did you never have a contract of any sort while you freelanced?

Thank you for your replies.

Just to try to make things clear...
I worked full time as a freelancer abroad (no contracts, just invoices), would like to work in the private sector in Germany as a regular employee and was wondering how I could prove to a prospective employer that I worked abroad should the need arise. I believe that Rebecca pretty much answered my question. Thank you.