If your key or organ player is friendly, will they throw up fingers using a "C" scale? C=1, D=2, etc. For flats, show fingers, fold then thumb. If they held up or If they showed two fingers, then folded and then a thumb, you would know that they were playing in Db. If two fingers, D. Ab would be 5 then 1 then a thumb. A full hand would be a G. What would a full hand, fold and then a thumb be?
Until you got familiar with picking up keys.