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.