With some deniers, you can catch this right away when they say something like, "Why are there still monkeys?" These are usually the people who haven't even bothered to do the most basic Google search on the topic and don't care to learn.
There are the smarter deniers though, and it often takes a while until they reveal some sort of gap in their understanding. One time I had somebody bring up "Junk DNA" and how biologists believed that we had a lot of info in our genetic structure that didn't do anything, and it turned out that they were wrong. The person was right on this point, but how does that affect the theory of evolution one way or the other? (Hint: it doesn't.)
The thing is, you can give a denier a comprehensive overview, using the most basic non-technical language, of what evolution is, and if they have a motivation (usually religious) to not believe in it, then it'll be like trying to convince the ocean to hold still. I'm not really bothering with that anymore, and instead I've changed my tactic. Oftentimes, it's best just to ask questions. You won't get immediate results, but if you can get them to start thinking a bit, then if there is any chance that they'll come around, that's the way it'll happen.
I like to ask the following questions:
What is it that you understand better about biology than the overwhelming majority of the world's biologists? Ultimately, that is what the denier is saying. A typical response will go along the lines of how scientists have been wrong in the past (often with examples from more than a hundred years ago). That's true, of course, and consensus doesn't equal truth. But I'm not going to drink pond water because scientists have been wrong in the past, so, you know, they could be wrong about the bacteria and parasites that are in it. In other words, that answer is a non-answer, and I point that out. Ultimately, they cannot give a reason as to why they understand things better than those whose life's work revolves around a basic understanding of evolution (which is what modern biology is).
What part of evolution don't you believe? You might get answers, but they'll only prompt more questions. It might also be good to break it down a bit and ask them which point is the one where they just don't accept:
- Do you not believe that genetic information is passed down from parent to offspring?
- Do you not believe in mutations? (If not, then what about blue eyes or people born with extra fingers? (I had a cat once who had two extra toes on each foot and an extra toe-less claw on each of her front paws.)
- Do you not believe that some mutations can be advantageous and enable a species to better survive?
- Do you not believe that these mutations would be more likely to be passed on?
- Do you not believe that these advantageous traits would alter a population over time?
- Do you not believe that these altered populations would bring forth a new species given enough time? (Because we have direct, observable evidence of this happening, so you'd have to show these to be hoaxes.)
With your more-informed deniers, they will often give you all of the above. However, they will often stop and say, 'Yeah, you get new species of lizards, but they're still lizards!"
From there, you can ask the following:
- Do you not believe that, given enough time and enough changes, you'd expect to see some radical changes? (Like reptiles bringing forth mammals? You'd expect to see things like the platypus if evolution were true, wouldn't you?)
- Do you not believe that domesticated cats and tigers share a common ancestor? (They share 95.6% of their DNA, and they're both cats.)
- Do you not believe that human beings and chimpanzees and bonobos share a common ancestor? (We share almost 99% of our DNA - more than house cats and lions.)
If they have no trouble accepting all of these, then proceed to stop the conversation and talk about the weather, as this person accepts evolution. If they accept the first two but not the third one, then ask them how that can be when we have more in common with our ape cousins than cats and tigers? (And the difference between lions and tigers is about the same as tigers and house cats.)
And while we're discussing DNA, why do they reject the consensus that the field of genetics proves evolution? And what about transitional fossils? (Some of them will flat out tell you that there are no transitional fossils, which is a bold-faced lie. If faced with this, suggest that they look up horse or whale evolution and then try saying that again.) And why the heck do I have a tail bone but no tail?
The best that you can really hope for is to get the person thinking. If you point out enough stuff, and they're genuinely intellectually curious, then you'll at least give them something to think about and you'll be pointing them in the right direction. The unfortunate thing with this issue though is that people don't approach it the same way as they would other issues. For some people, accepting evolution would mean tossing away an important part of their identity. This does not need to be the case though, as plenty of religious people accept evolution.
It's too bad, because the more I learn about it, the more amazing I think that the world is.