I hope your consider this:
A written contract between you and the adoptive party stating your companions rights to be provided for in a humane environment. Basically a description of how you expect your companion to be cared for and your right to verify this.
Request multiple references and verify them. I think the best reference may be the Veterinarian the potential adoptive party uses or has used in the past.
Request a home inspection. Perhaps the most important aspect of rehoming your companion to a loving family/person is the home inspection. Our environments are reflections of ourselves and the way a person lives will reflect the way they will care for your companion. I know a lot of people express concern about strangers coming to their home and I understand the reasons behind these concerns, however, I respectfully disagree.
I also recommend taking a photograph of you, your companion and their new family with the signed contract displayed.
I expect a lot of people will disagree with this advice but I ask you to consider this:
Would anyone with the intention to abuse – flip – or otherwise harm your companion ever agree to these requests? I don’t think so. The long term welfare of your companion is at stake so a little inconvenience really needn’t matter that much.