When does an economic theory become true?
It's just the same as in the physical sciences:

an economic theory, like any theory or model in physics or chemistry or biology, is wrong (H/T George Box), but it can be useful.

"True knowledge" is not a really a useful concept; not in physical sciences, not in social sciences. The important question is whether a model gives us useful insights, and whether it can constructively inform decisions.

An economic theory gains acceptance when it turns out to be useful.