The first problem with this statement is that it is impossible to know, even for the skeptic himself, whether or not the occurrence of "X" would actually result in his coming to belief. I am reminded of Gideon in the book of Judges. He asks God for a sign, God delivers, and then he asks for another sign. The second problem is that it presupposes that what God is primarily interested in is just getting people to believe He exits. The Christian response to this is two part. First, through creation and the moral law written on every human heart, God has given adequate evidence so that no one can say, "I would have believed if..." Second, God is not just out to have people believe He exists--even the devil and his angels believe. Rather, He desires for people to enter into a love relationship with Him.
With that out of the way, I want to examine three possible scenarios and show why it is foolish to say, "If God wanted to get me to believe, then He could do so very easily by just doing X." The specific examples also represent three general categories into which most of these sort of requests will fall.
"If God wanted to get me to believe then he could do so very easily by just...."
1. "Bringing my brother back from the dead." (Personal/Emotional request)
2. "Appearing physically in Time Square." (Large scale public manifestation)
3. "Causing 40 days of darkness upon my request." (Recreating an Old Testament miracle/Giving me power)
1. "Bringing my brother back." Suppose the skeptic is visited by his resurrected brother. His first thought will almost certainly be "How is this possible?" At this point, he will search for any way of explaining the phenomenon without accepting the miraculous. He will find multiple possible (although improbable) explanations. For example, it is possible that his brother elaborately faked his own death. It is also possible that his brother had a secret identical twin. Even more improbable, but still possible, is that someone could have gotten extensive plastic surgery, learned to impersonate his brother and studied important minutia about his life to seem convincing. If someone is determined to reject supernatural explanations, then he always assume a natural explanation more probable--no matter how ridiculous.
2. "Appearing in Time Square." One problem with God appearing physically somewhere is that we wouldn't necessarily know what to look for. Would He be in human form or would He manifest Himself in another way? A burning bush would be quickly dismissed nowadays as special effects. Another problem is that if someone predicted it, then people would dismiss it as having been elaborately staged. But if no one predicted it, then the vast majority of people would just dismiss it as a hoax--a hallucination or special effects. What if God appeared as a 90 foot Jesus in Time Square? Surely no one could deny that. Really? Even if thousands of people claimed to have seen him with their own eyes; and it was filmed and posted on every news channel, Youtube, etc; there would still be an overwhelming majority of the human race (billions of people) who were not there and would be very skeptical of such evidence in the age of computer generated effects and of tremendous illusions performed in public.
3. "40 days of darkness." The third scenario seems to like it would be the easiest for the skeptic to dismiss. Amazing coincidence? Sure. Miracle from God? Doubtful. Since it involves a natural phenomenon, there would inevitably be an abundance of scientific explanations (no matter how contrived) for the event in question. I feel certain that Al Gore and Michael Moore would rush out to make a movie about how the Republicans are to blame. Of course, there is the added miraculous element of the darkness coming "upon my request," but that would probably just be chalked up to coincidence. After all, how many millions of people are crying out for a sign at any given moment? Any number of them are bound to find something that they can point to and say "See! It actually happened!" No good skeptic would want to fall into that trap. And what if you weren't the guy who commanded the darkness, but just a good friend of his. Wouldn't you also want the power to cause miracles? Shouldn't God give you the same power that he gave your friend? But then what about your friends? You can immediately see the problem. If God just started handing out miraculous powers to skeptics--Bruce Almighty style--then everyone would feel justified saying "If God wants me to believe, then He should do the same for me."
I hope you can see from these examples just how foolish it is to say "If God wanted to get me to believe, then He could do so very easily by just doing X." Additionally, none of the above occurrences would, on their own, be enough to cause a skeptic to believe in the Christian God. They would bring us back to the Gideon scenario--asking for more signs. The skeptic might say, "Ok, now I believe some god exists. But if he really wanted me to believe he is the god of insert religion here then he could easily do so by doing X." What about the different variations of particular religions. I suppose God would need to provide a sign to clarify that. Where would it ever end? And if God could be bossed around and told what sort of signs to manifest for your approval, then what sort of God would he be? Would you worship a cosmic butler? No. That is why the whole complaint is utterly foolish.