The census should just ask it.
It does over here. And very interesting it is... but there are a couple of problems. First off it's too broad (ie; doesn't ask about seperate denominations with Christianity, Islam etc. And those with "no religion" come to think of it. It'd be interesting to see if the geography of Humanism was different to the geography of Atheism so to speak. I suspect it would be as well...) secondly about 10% or so of the population refused to answer the question and you can't make safe assumptions about them at all... there's a possibility that that tenth skews the figures off badly...
