Creationists love to try to disprove evolution, but they always miss the mark. Why is that? Well, the most obvious reason is that they simply don’t understand it. That’s what perplexes me so much. How can you hope to attack a scientific model you know nothing about? It would be like telling a Muslim that Islam is wrong because it forces its adherents to eat pork!
When I engage with Christians about specific doctrinal issues in the Bible,* I’m often told that I don’t properly understand Christianity. (This particularly amuses me when I’m using one Christian’s arguments against another’s. See also.) In my limited understanding of the Bible, I am told, I simply do not understand the greater context in which a specific passage (e.g., the pro-slavery ones) is meant to be taken. The Bible has contradictions, and each Christian handles these in their own way. Science works differently; because it’s far less individualistic, there’s less room. The scientific process works only thanks to a vigorous interpersonal process of bias-elimination, falsifiability, and objectivity. It’s a collective process, meaning that many minds contribute to the development of any given theory.
This makes it incredibly implausible that one ignorant creationist (and all of the young Earth ones fall under this category) will find “the one proof” that “evolution is false.” If you’ve thought of it, chances are that some biologist somewhere has already encountered the idea. If your criticism had any merit, the evolutionary model would be rewritten to account for it or discarded entirely.
Within the specific perspective of individual creationists, of course, it’s not all that surprising when they believe their criticism disproves evolution. This is because they are simply mistaken about what evolution is. What they are attacking (and indeed, probably successfully!) is nothing but a straw man—an erroneous construct that does not correspond to the actual position it is meant to refute (in this case, the scientific theory of evolution). If I were to suggest that all Christians are immoral because they are all pro-slavery, this would be a straw man because, while the Bible does advocate slavery, most Christians do not support this practice. Both Hanlon’s razor and personal experience suggest that creationists are not deliberately misrepresenting the theory of evolution; they have merely been grossly misinformed about what that theory says.
Jerry Coyne has a list of seven things that could actually disprove (or seriously discredit) the theory of evolution, and it’s a pretty good, concise analysis. In the event that you come across one of those ignorant creationists, this list can be a helpful thing in answering the question, “How would you know if you’re wrong?” These are concrete, evidential findings that we might expect if the theory of evolution were wrong.
How would creationists know if their beliefs are wrong? If the answer is “we can only know after death,” that should set off a pretty big red flag.
* As time goes by, I do this increasingly infrequently. Why? I've come to realize that most Christians do not actually believe in the Bible. Instead, they believe in their own personal interpretation of the Bible, which often actually just means the interpretation that their priest/pastor/whoever has told them to believe. As an aside, I'd also like to point out that an omnipotent god would be capable of guaranteeing that its holy book would be instantly and unambiguously interpretable so that anyone in the world would be able to precisely follow that god's mandates without requiring sixteen years of training in apologetics.