We know that we have found good plasterers when we have found them through referral by our friends and family and is also from http://www.rocksolidplastering.co.nz. We know that we have found a credible plasterer when they have honest and trusted credentials through the projects that they have done. It is also a plus when they already had worked for the same projects that are quite close to the kind of work that you have in mind.
Another plus is when they have found qualifications or even earned experiences to professional work and earn skills that have them earn reputation among construction firms. When the work especially started, you have to expect that people will come and go through your house either for work or for inspection. In the visits that they pay, you can see for yourself if they are friendly enough to negotiate with you or if they would work professionally.
You can also see for yourself about the kind of work that they have done through the projects they already have finished. Go visit them yourself and see how they perform with plastering and other forms of construction. When you have an enough knowledge about them, you can now decide if you will hire them or not.