Manual inspection, including real-time quality control using GIS software; quality control described in literature for the various regional studies.
Contrast that with what they say about NOAA's procedures for the same sort of data:
A long series of automatic quality control tests based on both statistics and physics (e.g., outlier tests, identical values two months in row, etc.)
Given the amount of data it's unsurprising that 'manual inspection' isn't enough.
Update There's a lovely bit of evidence from Professor Darrel Ince about software quality that I wholeheartedly agree with.