Truth is not determined by popular consensus - even when it's a popular consensus of experts in the particular subject.
However, I think that at least, when there is scientific consensus about a particular issue, the layperson should be given pause when his or her opinions contradict that consensus. At the very least, it should be the starting point for further research.
For instance, even though I'm not a biologist, I'm fairly good at explaining the basics of evolution. This is what I try and use when discussing the subject with somebody who doesn't believe it. While I am not averse to bringing up the fact that the overwhelming majority of scientists accept evolution, I never make a statement like: "It's true because most scientists believe it!" I just like to bring it up because if a person thinks that it's not true, even if they defeat me in a debate, they might have a tougher time with the people who really know what they're talking about.
I'm not quite as good at explaining climate change, so I have to rely on the point about scientific consensus a little bit more. Of course, you get people who dismiss that consensus, claiming that those who endorse the idea of anthropogenic climate change are only doing so because they have some sort of financial interest at stake. 'Cause, you know, those who deny it have nothing to gain by us continuing to use fossil fuels. Yup. No money in that.
As somebody who tends to get labled as a "liberal", I think it's safe to say that many of my like-minded friends are with me up to this point. However, there is a lot of concern about GMOs, and much of it seems to be coming from the same side as people who normally agree with me on the previous two issues.
I'm probably even worse at explaining GMOs - although I know that we're not really talking about Genetically Modified Organisms, as nothing we eat hasn't been modified in some way. What we're really talking about is Genetically Engineered Organisms. In other words, the crops have been altered in a lab rather than through artificial selection.
I have been suspicious about the concerns over GMOs for quite a long time. It began years ago when somebody tried to get me to sign a petition to ban them. As little as I know now, I knew even less back then, but the nature of his argument only made me question what he was saying. He had absolutely no specific examples of harm when it came to what GMOs can possibly do to us.
I've heard concerns from some of my more sober-minded friends that while they're not saying that GMOs are dangerous, they think that there needs to be more testing. They're also pretty big on labeling. (As of now, I'm in favor of labeling, but I'm skeptical as to what good that would really do.) In other words, they express to me that GMOs are still a big question mark.
If you find yourself in either the "GMOs are killing us!" or "We need to know more about GMOs!" camp, I would ask you to consider the following:
Does scientific consensus matter to you? If it doesn't matter on the first two things, then you're at least consistent if you disregard the consensus when it comes to GMOs. However, if they're enough to make you want to at least research the issue further and question your views, then you might want to consider the following:
The consensus of independent scientific organizations (not scientists who work for Monsanto or some other "evil" corporation) is that GMOs are safe.
There's a great article by Richard Green on the Skepti-Forum about the consensus and how one can avoid sources that are misleading about the issue. Some of the associations that have made statements on the safety of GMOs include the American Medical Association, the U.S. National Academy of Sciences, and the World Health Organization. The article (and this graphic) gives the specific statements that have been made.
Of course, this doesn't mean that GMOs definitely ARE safe. However, if you're going to say that they're not, or that they need more testing, and you're really interested in the truth, then you should look into what these organizations are saying and figure out why they're wrong.
As of now, I don't know if GMOs are safe or not. However, I'm humble enough to say that the people who know more than me seem to think that they are. At the very least, I should consider what they have to say before I consider the opinion of some random website that's selling snake oil.