You can't prove it, but you can persuade a reasonable person that they probably did. You could do this by showing them your processes, the UI as the user would

You can't prove it, but you can persuade a reasonable person that they probably did. You could do this by showing them your processes, the UI as the user would saw it, the code that ends up storing that in the database. Also, if your complaint rates are low, they will probably assume that the issue is not on your side.