It's not obvious what rule a journalist could use that is quick and easy to apply with no domain specific knowledge, would rule out taking creationism or MMR hysteria seriously, but allow taking cryonics seriously.
Compare the credentials of proponents and opponents. PhD in Christian Science vs in biology, "I have an autistic kid!" vs MD, and roughly equivalent on both sides, respectively.
Alternately, hire journalists with domain-specific knowledge. If you wanna compete with free media, have something to offer.
I just saw this link on a pop-news site.
It's a PDF file showing how good BBC's reporting of science news is in general, but more specifically it reports about the fact that the journalists give far too much credit to arguments from people who have no scientific backing for their argument.
http://www.bbc.co.uk/bbctrust/assets/files/pdf/our_work/science_impartiality/science_impartiality.pdf