...what does a 'Yaw string' look like?
Is it like a plumb line?
Not as such...
And what would I be looking for? I can see that if the aircraft was banked the line would be off centre, but would there be a displacement if there were yaw?
It's the other way round. A well coordinated turn would see the yawstring stay close to the centreline of the aircraft; too much/little yaw would show up as a displacement to either side.
On the 777 I bring up the flight control synoptic and see if there is any bias in the roll controls, e.g. consistently more 'up' on one side than the other, then tweak the rudder trim until the control surfaces are neutral (in smooth air) or averaging around neutral (turbulent). This should produce the least drag configuration.
This is with the autopilot in and not too close to a waypoint, so the aeroplane is trying to fly straight to start with.
I'd prefer a yawstring but Boeing don't fit them, for some reason. Maybe transonic wool is hard to come by?