For most plants a soil pH of 6 to 7.5 will allow the maximum uptake of nutrients which is a good thing. You can purchase simple test kits to use at home to get a pH rating on your garden soil. Alternatively some garden centres will run free tests on your samples. While you only need about a tablespoon of soil it is best to take samples from a few different locations around the area you wish to test, combine these by mixing then scoop up a good mix to use as your sample.
Acid, neutral or alkaline what is best?
Plants that grow naturally in peaty or alpine locations usually prefer a pH of around 5 to 6. Good examples are azaleas, camellias and blueberries. Plants that do best in high pH soils say 8 to 9 often include coastal dwelling varieties such as New Zealand Christmas tree, Cockys tongue (Templetonia) Grey Cushion Bush (Leucophyta).
So one answer is to use the pH of your soil as a guide to suitable plants. Its worth noting that the soil pH can vary quite a lot around one garden.