Basicially all comes down to experience.
Virgin have have engaged the services of ground crews with enough training to know all the dangers involved working around 737 aircraft.They would all be forte with the leading edge devices and the area's to avoid with running engines.NOT.
There has been 23 ground staff worldwide turned into mince since the 737 started ops .
I'm sure the same "trained" people doing turnarounds will notice possible/probable airworthiness defects.
To be candid a friend once asked what a guy was doing putting oil out of a blue can into the round things on the side of the aircraft.Obviously the engines were inside.
AN LAME,should we start a forum on the stupid questions asked over the years by the people now doing engineering tasks.
My experience of 25 years is that it is the smallest thing overseen that brings down the big bird.