Bob will accept that some phrase X is meaningful if there is a test that can be performed whose outcome value depends on truth value of X. If there is such a test, then we can construct a further test of asking someone who has performed the original test what the outcome of the test was. Since the people who set up tests are usually honest, this test would also be a test of X (provided the original test exists).

If I ask an honest peasant how long the emperor's nose is, but I also suspect no one has ever seen the emperor, how much do I learn from her statement? What if she says, "I have never seen the emperor, but other people tell me his nose is 5cm"? How many people has she talked to? Has any of them seen the emperor?

I don't know how to answer those questions, and yet your example is even less clear. You think no one has seen the emperor and you're not sure if he can be seen. 5cm? Well she is honest.