On a more serious note, I have no idea how you'd actually test this. How would you prove or disprove that someone experimenting with hallucinogens is actually communicating with something else?

The only thing I can think of is having two people under the influence being able to communicate with each other, with absolutely no other way of doing so. Subjects A and B have never met, and don't have any way of passing messages during their experience. Subject A is told to pass some randomly generated string to subject B, either directly or via some alien intermediary. If Subject B can guess Subject A's string, then that might be evidence of something interesting and objectively external happening.