In order to check the lateral position of the mast use the main (or
jib for fractional
rig) halyard and swing it to the port and starboard rails.
Don't forget to sight up the mast to look for bends. The lower shrouds can be used to straighten some of the bends. Often times the lowers are set looser to allow for mast bend. In fact in some racing classes the lowers are adjusted according to the wind speed.