How can I legally prove that I have recompiled the Ubuntu source?

I like to create my own distro based on Ubuntu. I may modify the source of some packages. When I read intellectual-property-policy of Ubuntu, I noticed the following rule.

Any redistribution of modified versions of Ubuntu must be approved, certified or provided by Canonical if you are going to associate it with the Trademarks. Otherwise you must remove and replace the Trademarks and will need to recompile the source code to create your own binaries. This does not affect your rights under any open source licence applicable to any of the components of Ubuntu. If you need us to approve, certify or provide modified versions for redistribution you will require a licence agreement from Canonical, for which you may be required to pay. For further information, please contact us (as set out below).

^{Source: https://ubuntu.com/legal/intellectual-property-policy}

Consider that I have recompiled the Ubuntu package source and built my own repository: I actually pull the source apt-get source, build deps apt-get build-dep and use dpkg-buildpackage to compile. Once .deb is generated, I host it in my local private apt-repository.

In the future, if I have to legally prove that I have indeed recompiled the Ubuntu package source, how can I prove it? Do I have to inject any custom tags in generated .deb or binary inside the .deb? I checked for the custom tags (just to find some leads) in MintLinux distro's .deb but I couldn't able to see any custom tags.

You have perhaps misread the sentence.

Recompiling your binaries is a technical requirement (not a legal requirement). Your source code changed when you removed all the trademarks, so of course you must recompile that software.

The only "proof" you need of non-trademark-infringement is that folks cannot find any Ubuntu trademarks in your source nor in your binaries.

Keep in mind the proper source attribution may be required by the software's license, so don't just do a global search/replace that might obscure or mangle the copyright holder, author, upstream source(s), code comments, changelogs, etc. License violations will get you socially snubbed, and perhaps into serious legal trouble, too.

If you plan to have your own repository with unmodified Ubuntu packages, then you don't need an approval from Canonical.

If you distribute modified packages, then you have to provide your source code, e.g. patches.

If we are talking about deb packages, the patch section with appropriate changelog is sufficient.

If you want to make a whole modified distribution available, you need to remove all trademarks from it. It doesn't mean that you have to remove all ubuntu words from everywhere though.

BTW I AM a lawyer, not a copyright one though. So this is not a legal advice ;-) Anyway I can understand how GPL works.