Really good question! I think people use experience. They consider a belief system different to their own, and then life throws up events which confirm that belief system to be compatible in some sense with reality.
The problem is that just about any belief-system can offer up its own validations because our minds interpret things according to the maps we learn about - not the other way around. So for example, when i first read about communism and the idea of oppression of the working class, suddenly i started finding evidence of it everywhere. The same goes for every type of belief-system, including metaphysical ones.
Talking about metaphysical "truth" is a bit of play on words for me. And yeh, i do believe it is more important - the usefulness of the belief-system for you - than the evidence itself. Perhaps when defending "the evidence" it actually means defending the usefulness of paradigm.
In this way metaphysical truth might be decided by that which gives comfort or hope or happiness or satisfaction of emotions, or courage to do something etc. A tool.