Choosing my name as an author when publishing a scientific paper, can I use my "unofficial" first name? I have one official first name which appears on my ID card. I have another first name which friends call me by. Is it possible to use my unofficial first name and official family name to publish scientific papers? How can I prove to someone that I am one of the authors?
It depends on the field, and in particular on the venue in which you are publishing. It's possible that some journals and conferences might have policies about this, which would of course override anything you read here. But in general: nobody checks (or cares) whether the name you put on a published paper matches your official name. So if you use an informal variant of your name, e.g. if your name is "Stephen" but you publish as "Steve", nobody will bat an eyelid. If you use a nickname or publish under a pseudonym, it would be a little odd if the nickname is something that sounds very informal, but still, it probably won't cause problems.

What people do care about is building a record of your work, and putting a face to the name if they know your face. So you should (1) be consistent with the name you use to publish, and (2) make sure other people in the field know that it refers to you.