Adam Clarke on John 6:30

What sign - Τι σημειον, What miracle; so the word is evidently used, Joh 2:11, Joh 2:23, and in many other places.

That we may see, and believe thee - That, having seen the miracle, we may believe thee to be the promised Messiah. They had already seen the miracle of the five loaves, and did not believe; and it was impossible for them to see any thing more descriptive of unlimited power and goodness. Even miracles themselves are lost on persons whose hearts are fixed on the perishing things of the world, and whose minds are filled with prejudice against the truth.