The answer is predictability. We see things. We don’t know what they are or what they mean. We come up with theories about what they are and how they work. Those theories in turn let us make predictions about other things or behaviours we should see from similar things in the future.
If our predictions are correct, then we know our theories are good at describing what we see, and we thus have improved our understanding of how the universe works. Even if our predictions aren’t correct, we have learnt something.