That's been my point since the day I arrived here at JREF.
In short, they can't.
The messages are political and media driven. We have extremists on both sides (I ignore both btw) hijacking information and we have ideolists doing same to further their political ends.
We have scientists fiddling data and their detractors doing same.
Alarmists going 'one up' to get the headline and make a name for themselves with seemingly no one to stop them and their 'warmer' friends remain silent to these lies (goes the other way too btw).
Joe Sixpack has no chance sorting the wheat from the chaff.
And why should Joe believe one thing and not the other?
Why should Joe believe this expert ahead of that expert?
It is confusing.
Now, Joe votes - and the politician wants to keep his job, and the bureaucrat wants to please the politician, and the scientist wants to keep getting his funding from the bureaucrat - and so Joe's vote is important.
There is climategate, hockey sticks, alarmist predictions not coming true (time after time after time), wikipedia, and the IoP memo, IPCC errors and Pachuri's pornography to name but a few, and we ask..
How is Joe supposed to evaluate scientific claims?
So it's a good question and one I have been asking all along.
The only answer I see is to not let the activists and politicians get involved until the science is actually settled. Smarter people than me must be able to come up with balanced, better alternatives
Sadly that will not happen so ultimately, valid science, the public and the opportunity for genuine social reform are the losers.