How do I know where the direction of force / thrust will be applied by flemings LHR ?

Hold you left hand so the thumb, first finger and second finger are perpendicular (like x y z axes). See picture in link.

Rotate your hand/arm so that:

your First finger points in the direction of the magnetic Field (which is from the north pole to the south pole of a magnet if tou need to woprk it out);

your seCond finger points in the direction of

the Conventional Current (which flows from the positive terminal of the supply back to the negative terminal if you need to work it out);

then your Thumb points in the direction of Thrust (motion), which is the force tending to make the wire move.

See picture in link.