Scientific theories are True if they are Useful
Meta-modelism.
Science doesn't actually operate within a "true or false" paradigm. Rather, it works with scientific theories, which are models: simplifications of reality that help us understand one aspect of it at a time. Because reality is far too complex, chaotic, and analog, it is impossible to work with it in its entirety. So these models we work with, being simplifications of reality, a kind of "slices," are obviously "true" up to a certain point, in the sense that they correspond to certain aspects, beyond which they become false, no longer corresponding to other aspects.
Of course, a scientific theory must first and foremost be valid and reliable, but this isn't measured by whether it is true or false. It's measured by whether it is coherent, internally consistent, and repeatable.
Scientific theories, these "models," must simultaneously meet two contradictory goals: they must be simple, and therefore easy to work with, and at the same time complex, so as to simulate reality as closely as possible.
All right, then how do you measure the value of a scientific theory? How "true" is a theory? Is Euclidean geometry truer than non-Euclidean geometry? Is the theory of relativity truer, or string theory? Is the Big Bang model truer? Is the creationist theory true, the one where Adam was made from clay, belly button and all?
It doesn't matter so much how True a theory is.
A theory has to be functional. The right question is how useful a particular theory is in a particular context.

