I sometimes worry about whether something is right or wrong, good or bad, and I don't necessarily want to blindly take anyone else's word on what's right and wrong, not even God's, or rather that of those who claim to speak for God. I wanted a metric by which to measure their counsel for myself, and I wanted it to be something sturdier than just a gut feeling about what's good and bad. Fortunately, I think I've found one. One way to measure the goodness or badness of anything (and perhaps anyone, for that matter) is to what extent it (or they) contribute to or diminish from human happiness. For example, trees are good because they make people happy, and because they produce oxygen, which facilitates human life, which is necessary to human happiness. Conversely, car crashes are bad because they diminish human happiness, and thus reckless driving is also bad, because it increases the risk of car crashes, which diminish human happiness.
This isn't a foolproof method, mostly because happiness is difficult to measure and because it's not always possible to predict whether an action or course of action will lead to happiness or unhappiness. In most cases, we just have to eyeball it and take our best shot at increasing human happiness. Still, I think it's a half-decent way to measure how good or bad something (or someone) is. Does it (or do they) make people happy of unhappy? How happy or unhappy does it (or they) make people? How long does that happiness or unhappiness last? Eating right and exercising, for example, might diminish human happiness in the short term, but it diminishes unhappiness in the long term. Thus, eating right and exercising are somewhat mixed, but mostly good.
Another complication is the existence (or nonexistence) of an afterlife. We have faith in the afterlife, just as we have faith in Jesus Christ and the prophets, but faith is not a perfect knowledge, and other people have faith in their religions, too. We can't all be right. Given that we don't have a perfect knowledge of the afterlife, it's not possible to say for certain which actions will lead to happiness of unhappiness in the afterlife (assuming there even is one). However, we can make an educated guess. Most religions believe that those who are good will be happy in the afterlife. Thus, if we want a happy afterlife, we should do good here, and we can measure good here by whether it increases or diminishes human happiness. If we contribute to human happiness here, we're probably making good choices, the kind of choices that lead to a good and happy afterlife, or at least a good and happy mortal life.
So, I would say that things (and people) are good if they contribute to human happiness. Thus, we can do good, and be confident that we are doing good, by doing things that help people be happy and find happiness. If we have found happiness in the Gospel, then it is good to share that happiness by sharing the joy of the Gospel with others. If anything in our life has brought us misery, we can do good by warning others against it and the unhappiness it can cause. The Gospel and the teachings of the church can give us more specific ideas of what things are good and bad and what things lead to happiness or unhappiness, but I believe that this is a good rule of thumb to consider in cases not completely covered by the teachings of the church, and it's a good way to confirm some of the truths that are taught in the church.
If it contributes to human happiness, it's good. If it contributes to human unhappiness, it's bad. It could very well be as simple as that.