I generally do it by feel, partly because I'm treating "value" somewhat subjectively, something like "intellectual value" or "scholarly value" rather than "number of hits". So I look for things that Wikipedians don't seem to be spending a lot of effort on, but which imo a good encyclopedia should cover.
This sounds quite interesting. Do you do this by rough intuitive feel, or is there any quantitative/analytical data from Wikipedia that can be used to prioritize attention this way? (For example: a list of places where the imbalance between readers/inbound-Google-referrals and content/active-editors is greatest?)