If the example of Jonah was supposed to show that God speaks directly to people, and not through others, well, that's just one example. There are many cases of a prophet passing God's message along to others, usually large groups of people, and expecting them to accept it (and not often, if ever, are they expected to get a confirmation before they believe it).
Paul, though, in
1 Corinthians 14:37 is expecting others who think they have spiritual powers to acknowledge that he is passing along commands on from God. Bit of a taunt, maybe? So, perhaps this is an example of that confirmation you mention (as I asked about in my previous post). But it certainly isn't the norm. Most often, a prophet just expects everyone to believe him.
Sometimes, signs and wonders are supposed to give support to a prophet's claims. Aaron's rod turning into a snake. Elijah putting the prophets of Baal in their place. But, alas, false prophets
also perform signs and wonders. We are not really told what to do with dilemma. (Unless you think
1 John 4:2-3 comes to the rescue.)