How can I prove that I am a legal DDK user if some body asks ?
Who do you think will ask? You wouldn't have your copy if you did not have 'Agree'ed back then when you downloaded it like I did. I know I had to click 'Agree' at the bottom of a long text before I got to the page where I could download the files. In theory IBM may have recorded my IP but I think no one will and can trace back which IP the company used about 15 years ago when I downloaded it. Maybe I had to register with an email address but I don't know anymore.
You do not have to prove that you do nothing illegal. It's the other way around - you are not guilty until someone proves you are. That's the way the justice works in civilized countries (of course we know some arguments from some people are taken more serious than others but that's hardly related here).
Anyway you're restricted to the license you 'Agree'ed independently if you still remember the wording. I don't. But I'm pretty sure it does not allow me to share copies of the sources but I can build my own drivers with them and share the binaries. The sources include -
/* You may use this code in accordance with the IBM License */
/* Agreement provided in the IBM Developer Connection Device Driver */
So if I ever would share my sourced build upon the DDK I would have to search for that.
Btw. my copy does not include a license other then the note above.