We probably can't observe the multiverse. What we can do is postulate how events might play out if, as is suspected, subatomic particles have some interaction with their counterparts in "nearby" universes. We can model the various possible explanations and use the models to generate testable predictions. Assuming that process ultimately rules out some of the models and favors others, we still won't know that there really are multiple universes, all we'll know is that a model that assumes there are, and assumes they interact in specified ways, is a good explanation for the observed phenomena. "Good" in the sense of being as simple as possible, and no simpler, hard to modify without contradicting observations, etc. Other explanations might work, but just be conceptually weirder and harder for us to think about.
Or maybe we'll eventually find something in the model that demands a multiverse, and for which we can find no other simple, consistent explanations. That will tend to convince us that it really exists. Or maybe the theory will show us a way that we can scale up the interactions to the macro level, where we can observe the multiverse. Or maybe we'll find something that contradicts the multiverse... or maybe none of the above will happen and the whole concept is just a game for very clever people.
Many things are possible, that's why it's fun :-)