Hi,
first the correct wording should be "de-icing" at that stage...
And for your question, you have to thrust de deicing staff...
the guy in the nacell of the deicing truck has a accurate sight ot the situation..or sometime..when no guy 's up, a camera send pictures to the operator in the truck..
option: If you don't thrust them..have a ladder and check yourself...
specially at the holding point, almost ready to go...