Well, I'd use pax numbers as a good first approximation of viability - the data is available, as s7s has kindly shown. If I wanted to get a tad more sophisticated, I would look to look at the total and high income population in the airport's catchment area - I might try to get some data on house prices as a proxy for the latter. I would also look at time/distance to next nearest airport and identify island airports. I would also include freight usage as an additional source of airport revenue. Put all into an econometric model, place in a warm computer and cook the figures until ready.
That is assuming that airports with perhaps limited passenger numbers (due to a small or less prosperous catchment for example) would not be viable without those passenger flights. I would bet that many of the airports listed as 'regional' and 'marginal' by s7s do not rely too much on revenue from passenger services, particularly those in the 'marginal' catagory. I suspect many are commercially viable even if they dont appear so on the CAA passenger statistics.
On regional planning, let's look at the potential value added of intervention compared with the market solution. How might markets not give a socially optimum solution? If there are two marginal airports and only enough business for one: both will close unless they either come into the same ownership or there is intervention. If the airport land is temporarily more valuable as housing but closure would have a impact on the regional economy that cannot easily be monetised. if our friend the megagarch decides she wants airport A to stay open, even though it is in everyone else's interest to keep airport B. (Of course, in this case, she might flounce off and buy Italy or something).
Would that not then require state or local authority intervention? Is this not also unlikely considering most local authorities have distanced themselves from the running of regional airports rather than trying the opposite?
The Government washed their hands of regional airports years ago and formed BAA (for those deemed to be of national importance). They believed that local pride, and a better knowledge of the local area would be a better driving force behind the regional airports. How likely are they to ever change their attitudes?