Well, that's the $64,000 question and I will admit it isn't easy. But since I can't read minds I take the position that unless they give me reason to beleive they are NOT serious (i.e. something in the context, past history of the speaker, etc.), I assume they are serious.
Which is indeed the problem. The likes of Coulter merely spout slogans to fire up a mindless base--see Jennineianine Garglcumafloro--see what I did there?--and others on both sides. They have too much to lose--$$$--to actually act.
But you never know.