Great question, and one that humanity has been struggling with since its inception! Moral statements, such as "murder is wrong," are more challenging to prove true than something objective, such as "the sky is blue." For objective statements like the latter, we use a truth-discerning system of inquiry known as science!
Science was developed to distinguish truth from falsehood. It uses observation, logic, and repeated experimentation to test hypotheses, separate variables, and ultimately develop theories about the world. In the parlance of scientific inquiry, however, a theory is not a guess about something; instead, it is the simplest and best validated explanation for a specific phenomenon. Theories are the best way to approach truth, but they are not truth itself; science acknowledges the possibility that new evidence may contradict a previously established theory and thereby debunk that theory. Because of this, no theory is considered true.
Consider the theory that the Earth is round. We begin by defining what we mean by "Earth" and "round" before considering the implications of this categorization. If the Earth is indeed round, one may conjecture, wouldn't that mean that a ship moving away from me towards the horizon would slowly drop from my view? The next step one would have to take would be to perform an experiment by watching a ship as it crosses the horizon and observing whether or not it does actually drop from view. If it does, then the theory is valid. If it does not, then a new theory must be devised. Though we cannot conclude that we have discerned objective truths through the inquiry process of science, this process does help us distinguish fact from fiction and therefore allows us to approach objective truth.
I've posted the link to the eNotes science page to help you understand how science operates as a truth-seeking process. I hope this helped!