You can't. People minds are dynamic systems. Even if you trust a human now, you can never be sure what changes will occur in the future to make that human choo

You can't. People minds are dynamic systems. Even if you trust a human now, you can never be sure what changes will occur in the future to make that human choose differently then expected based on previous experience - child sickness, family troubles, hormonal disturbances, environmental toxicity, parasites, whatever really ... anything can influence human behavior in radical manner. Notice that time here is relevant. Given small enough time scale, you can definitely trust people. And vice-versa. So, the question is not how to trust people, because you can't trust anybody given enough time, the question is how to plan things in your life so that broken trust isn't detrimental for your status.