Primarily on recommendation - either from a satisfied client or your architect. Even then everyone has their "off" jobs and you can be unfortunate. I currently have clients who employed ( who turned out to be) the builders from hell - on the recommendation of their architect. Its very nasty and very expensive at this stage.
hi
check out ratethebuilder.ie,alternatively demand to see previous work done by the builder,but make sure you get to speak to the owners,not just look at the house.they could be on holidays!my builder actually did this while the owners where in portugal,he did'nt actually build the house!
be very careful and the best of luck with it.