To gain trust of your audience, you can first help them in online forum / slack where they hang out online. You can start by answering question they ask, and take note of the question and "why" they asked this question (eg: why do they ask this on the forum? what are they trying to achieve). Once you have helped enough people, and visibly online in public, you will start gaining trust of them, and you would notice pattern of the common question asked by your audience (which means reoccuring pain), then you can start building a product based on this pain, and you can be confident that this would sell as this actually solve a pain the audience is facing.
A shortcut for finding an audience which you can help quickly, is to choose audience which have the same occupation / profession as you.