Check that the plumber holds the right licence for your state or territory, and ask for the licence number so you can verify it with the relevant building or plumbing authority. Get a written quote that explains labour, materials, call-out fees, GST and any potential extra charges before work starts. Ask whether the plumber has appropriate public liability insurance and request details of any required permits or compliance certificates. For major work, compare a few quotes and use the written scope to discuss changes with the plumber before approving them.