You tell me that you saw a beautiful sunset last night. I conclude that you saw a beautiful sunset last night. You are talking about Mother Teresa. I conclude that you won't say that she was a sneaky politician. You promise to bake a pie for the party tomorrow. I conclude that you will bake a pie for the party tomorrow or you will have a good reason for not doing so. I tell a graduate student to read a page of Kant for next class. I conclude that she will read a page of Kant for next class or will have a good reason for not doing so.
All of these are inferences of an "is" from an "ought". You ought to refrain from telling me you saw a beautiful sunset last night, unless of course you did see one. You ought not say that Mother Teresa was a sneaky politician, as she was not. You ought not fail to bake the promised cake, unless you have good reason. The student ought not fail to read the Kant, unless she has good reason.
All of these are of a piece. We have prima facie reason to conclude from the fact that something ought to be so that it is so. In particular, belief on testimony is a special case of the is-from-ought inference.
In a fallen world, all of these inferences are highly defeasible. But defeasible or not, they carry weight. And there is a virtue—both moral and intellectual—that is exercised in giving these inferences their due weight. We might call this virtue (natural) faith or appropriate trust. We also use the term "charity" to cover many of the cases of the exercise of this virtue: To interpret others' actions in such ways as make them not be counterinstances to the is-from-ought inference is to charitably interpret them, and we have defeasible reason to do so.
The inference may generalize outside the sphere of human behavior. A sheep ought to have four legs. Sally is a sheep. So (defeasibly) Sally has four legs.
I used to think that testimony was epistemically irreducible. I am now inclined to think it is reducible to the is-from-ought inference. Seeing it as of a piece with other is-from-ought inferences is helpful in handling testimonial-like evidence that is not quite testimony. For instance, hints are not testimony strictly speaking, but an inference from a hint is relevantly like an inference from testimony. We can say that an inference from a hint is a case of an is-from-ought inference, but a weaker one because the "ought" in the case of a hint is ceteris paribus weaker than the "ought" in the case of assertion. Likewise, inference from an endorsement of a person to the person's worthiness of the endorsement is like inference from testimony, but endorsement of a person is not the same as testimony (I can testify that a person is wonderful without endorsing the person, and I can endorse a person without any further testimony). Again, inference from endorsement is a special case of is-from-ought: one ought not endorse those who are not worthy of endorsement.
If is-from-ought is a good form of inference, the contraposition may-from-is will also be a good form of inference. If someone is doing something, we have reason to think she is permitted to do it. Of course, there are many, many defeaters.
It is an interesting question whether the is-from-ought inference is at all plausible apart from a view like theism or Plato's Platonism on which the world is ultimately explanatorily governed by values. There may be an argument for theism (or Plato's Platonism!) here.