Scrotometer.
the main reasons for using 30 instead of 40 are better approach climb gradient ie higher landing weight and/or lower minima and also better control in strong x-wind landings
There is no such direction in the Boeing FCTM about using Flap 30 in lieu of Flap 40 at the max crosswind limit. Handling is the same technique for both.
The difference between all engines climb at Flap 40 and Flap 30 on GA is negligable because the first thing after GA thrust is to select Flap 15 regardless of the Flap 40/30 landing flap. The main reasons for Flaps 30 landings is it saves 20 kgs of fuel over Flaps 40 when measured from the outer marker to threshold. Also slightly less noise footprint.