(2014, July 17) Multiplayer Detect Sky

/scoreboard objectives add AirTest dummy
/scoreboard players set @a AirTest 0
/stats entity @a set AffectedBlocks @p[r=0] AirTest

/execute @a ~ ~2 ~ /fill ~ ~ ~ ~ ~99 ~ <BLOCK> 0 replace air

/execute @a ~ ~2 ~ /fill ~ ~ ~ ~ 255 ~ air 0 replace <BLOCK>

/say @a[score_AirTest=99,score_AirTest_min=0] is Covered.