It was easy at Sumburgh when I did my 30 day 'penance' there back in '72.
The airfield was only just above sea level; there was a cliff called 'Fitful Head' about a mile north west which was just under 1000ft amsl. You just looked at Fitful; if the cloud was below the top it was obviously below 1000ft (say 800ft) and it was in the clear, it was something like 1200ft.
Most larger airfields nowadays (and some smaller ones) have an automatic weather station which, by means of lasers, assesses the base of multiple layers of cloud, which can then be entered on ATIS, sometimes automatically sometimes by an observer reading it out.
Last edited by chevvron; 21st October 2012 at 18:17.