There's no specific check for the fan speed, u just have to feel that enough air is blown thro the radiator. u may compare it with some other baleno or ask a mechanic about the speed. if the fan speed is slow than on slow speeds there isnt much airflow thro radiator "as u have also mentioned that on hi speeds temp is ok", and the temp raises which also causes the chkengine lite to come on. or the AC condenser fan speed is also not rite, then a part of it also heats the radiator.
btw did u get the head facing also or not. if yes, then may be it is not properly faced which is causing this effect. a slight error in that will not cause water and oil mix, however, may cause this problem.