For one to know if an action is pure or of moral worth, one would use the formula, the rule; categorical imperative. Kant lays out this rule and in every given situation you would apply the categorical imperative rule to determine your course of action, what one ought to do. .
Moral ideas are universally valid, only if they are based on the intrinsic validity of a pure concept. Pure or " a priori" concepts are ideas that occur to us when we think about things in our minds, prior to and independent of any experience of how things happen in the world. "Empirical" or "a posteriori" concepts are ideas that we derive from out experience of the world. When one thinks of what they ought to do they base their reason in their morality, moral laws. Kant's moral law is reason and moral laws are suppose to be undertaken for the sake of morality alone; we are suppose to have a pure motivation and not self-interested. .
A pure motivation is to have good will, a duty. Duty is what is behind our motives, reason and desires. To have an immediate desire to be good and deal honestly in all situations is considered one to have moral worth; this is our duty. Kant offers three ways to know if ones actions are not considered to be pure, to have moral worth and if the action is done solely out of duty. First, if one is only concerned with the sake of duty and that is their only reason and motive and second, if a good will is only performed out of selfishness and third, if one only helps another because they feel sympathy for the individual. To know if ones duty has moral worth is to ask you, "Would I still act in the same way if a moral law didn't exist?" If you answer, "Yes", than it is of moral worth, it is pure. .
Kant says we have a freedom of choice; we always have a choice. Reason is probed by our will and will is probed by our inclinations. We have a choice by having control of our will and that is done by reason.