In response to such a person, I would point out that faith in God is supposed to depend not on evidence and proof, but rather on belief.
In today's world, Jesus does not walk the Earth and perform miracles. Even if something miraculous occurs, there is no direct proof that it is a miracle that has been performed by or through God. Therefore, it is pointless to expect to be convinced by miracles. A person either believes or does not believe.
So I would respond to such a person by saying that there is no way that we can ever prove the existence of God and by saying that a person with true faith believes regardless of proof.
Miracles can persuade a person with clear and clean mind only. A prejudiced person will say that he/she is waiting for miracles to happen, and when they do happen, he/she will say it is magic. It has been their attitude in the past and you have many examples of the same in the Scriptures. The miracles of Moses to Pharaoh and his reaction is known to all the believers.
God can be seen everywhere if some one has the eye to perceive Him and it is a matter of belief.