Max Tegmark's Many Worlds paper (quant-ph/9709032) gives an experiment called quantum suicide for testing the Many Worlds Interpretation. It seems possible a variant of this experiment could be used to test the idea that all universes exist (is there a shorter name for this idea?).
Basicly the experimenter reduces the measure of his personal continuation in this universe sufficiently so that his mostly likely continuation is in another universe. This would involve setting up a suicide device that is almost certain to succeed in this universe but not as likely to succeed in another. However, this experiment is even less practical than quantum suicide since constructing such a reliable suicide device (I guess failure probability must be much smaller than 2^-100) would be nearly impossible.
