For all of the evidence we have that things evolve over time, there is still enough evidence extant to support the old saying that "some things never change." Just as in the days of old, folks are still demanding a sign from God - some proof that He/She/It exists and/or has ever had any contact with us. And these folks aren't interested in just any old sign, they want a sign that meets a standard or criteria which they have chosen. In short, we want proof!
The problem is that science can study and test natural phenomena, but what do we do when we run up against supernatural phenomena? We can deny its existence or attempt to use natural phenomena to explain it. We can accept it based on our own observations and experiences. We also have the ability to accept things based on belief (faith). For some folks, a voice from heaven or a miracle would do the trick. For some folks, a group of scientists being able to perform tests and experiments in a controlled environment (with the ability to replicate the results) would be acceptable. Whatever the personal preference of the individual, the truth is that most of us demand some kind of sign from God. Hence, in the light of these observations, I think it only fair to acknowledge that it would be very hard for God to provide us with any sign that would have universal appeal. What would be enough for you might not be enough for me!
According to the Judeo-Christian Bible, God (YHWH) has given mankind a number of signs over the course of the "history" of "His" interactions with mankind. For the Hebrews, the Sabbath was a sign between the people and their God. (Exodus 31:12-17) This same God also offered a sign to the House of David in the form of a virgin giving birth to a child that would be called Immanuel. (Isaiah 7:14) The Gospel of Matthew records a couple of instances where people demanded that Christ give them a sign. In one of those instances, we read: "The Pharisees and Sadducees came to Jesus and tested him by asking him to show them a sign from heaven. He replied, 'When evening comes, you say, 'It will be fair weather, for the sky is red.' and in the morning, 'Today it will be stormy, for the sky is red and overcast.' You know how to interpret the appearance of the sky, but you cannot interpret the signs of the times. A wicked and adulterous generation looks for a miraculous sign, but none will be given it except the sign of Jonah." (Matthew 16:1-4, NIV) In other words, "you cannot interpret the signs you have - what good would it do me to give you more of them?"
I think that the same thing could be said of us today. If you can't see God in the world around you, then it isn't very likely that you are going to accept any other sign or proof of "His" existence. If you can't see God in the phenomena that we call love, then you are not likely to accept any miraculous signs or apologies from some theist. If you haven't personally experienced the supernatural, then you aren't very likely to be swayed by a voice from heaven (After all, it could be a recording of Morgan Freeman's voice being broadcast by a drone that has been obscured by a cloud). From a Scriptural perspective, I'm sure that YHWH would say that "He" has given us many signs - we simply didn't like or accept them. If you think about it (even if you don't approach it from a Scriptural perspective), what is God's incentive for providing us with a sign? Would we all drop everything and rush to "His" side? Would we all suddenly become believers? Would the whole world convert? What would we convert to? Next on the agenda: "Give us some direction/instruction!" This is starting to sound circular isn't it?