How to use breastpump

FTM here, i have yet to deliver. Ive bought both manual (medela) and electric wearable breastpump. However am still not exactly sure the right way to use it. I read its advisable to use the Massage mode first to stimulate milk or a let down then followed by expression mode. However in the situation whereby we need to pump at office or outside, when using massage mode first, how do we know theres already a let down ? Since we wont really be able to see the cup from top view as its covered by the motor. Will we feel milk coming out or something ? Sorry if its a stupid question. #FTM