My understanding was that many-worlds could be tested experimentally if we were able to set up large objects in superpositions, and I thought that there is no reason to expect that it isn’t physically possible to do so (we just don’t know how at the moment).
You can’t get any information from that because the main interpretations all make the same predictions from an outside perspective, which is what you’ll expect to see.
Besides to put a person in a superposition may require a machine as large as the universe, so you get issues with the speed of light.
The only way to test MWI is multiverse immortality - many worlds means you should expect to always have some future experience - there is no real death.
