One possible answer is that these statements correspond to moral facts. But how do I measure a moral fact? Where are they? Can I touch them? Feel them? Taste them? See them? Hear them? This is a problem.
Another possible answer is that value statements like "x is good" or "one ought to x" correspond to empirical statements like "x produces happiness" or "doing x produces happiness". As we will see this solution has problems too. Ayer's answer is that value statements/judgments only reduce to the attitudes of "hooray" or "boo", not empirical statements.
This scenario is an example of using speculative reason which relates to empirical facts about the world. If you are so inclined you can test your statement "following the trail of blood will lead to the criminal"; that is, you can test whether the content of the statement matches up with the real world. Also notice, that this type of speculative reason allows you to make predictions that can be verified, i.e., "if I follow the trail of blood, I'll find the criminal".