While a great deal of Agnostics may be thinking they are serious, they are not. But for those that are serious Agnostics, i.e. thinking that there is a God, but Jesus may not be the son of God, or something like that, it's a form of relativism. In other words, what suits you.
Trying to establish a "proof of agnosticism" is wildly ironic and super hilarious to me. An agnostic does not need to establish anything in order to be considered valid. An agnostic just is because of the way they believe in things. There does not need to be any proof of validity.
What agnostics basically say is that they do not know about the truths of this world and what lies behind it. So there is nothing to prove. That means that the theory of agnosticism is always a valid one. It is the classic human stance of being willing to find out later.
It is very easy to prove agnosticism. Just look around at the world at all the different religions, all who believe they are the true religion and you see proof. The answer is we don't know which is the real religion and if there even is one. So if you are agnostic you have evidence of why.
The key issue here is that "True" agnosticism is nearly impossible. However, many people do not take a definite stance for either theism or atheism. Thus, Pantheists and Deists tend to call themselves agnostic while truly leaning one way or the other. Someone who is truly 50/50 or believes that they cannot know must partake in a serious mental juggling act simply to maintain their lack of view.
No, there is not proof that agnosticism is valid, because humans do not always know God's plan. It is awfully arrogant of a human to think that they know better than the creator of the universe. It is hard to do, but people need to trust in God's plan. Agnostics want to believe themselves, when they are just people, and God is God.