You can’t prove that someone learned …but maybe you can put the onus on them to document it.
People need to know if someone completed a learning experience. That is a binary question: Completed y/n. What question can be asked that is binary and relevant to learning?
What we really want to know is can people use what they learned. How about setting up a group in the enterprise social media tool that only consists of statements from learners about how they used what they learned?
If you are willing to commit to saying you learned under the scrutiny of your boss and peers, then you probably learned. Let the post send a TinCan API call to an LRS that can mark the person complete in the LMS. Done!