This would seem the most prudent approach, particularly if there is a physical extraterrestrial involved. Could such a being defeat the test if it understands what is being done and is asked directly by the channeler to submit to a test? Could it even be dangerous for the channeler to ask such a being directly, to perform a test of authenticity?

There can be dangers in confronting a physical extraterrestrial with a challenge. This could distinguish you as a troublemaker who has greater freedom than most humans to ignore or surmount extraterrestrial doings and there will be a follow-up to find ways to constrain you. If you keep this an innocent-seeming exercise and do not show off what you know, but allow the divine realm to assist behind the scenes and convey the right information you need, that will be a far better approach and can, in fact, be done.