Take the test

First, you should establish whether your sensor actually needs cleaning. Although cleaning doesn't have to be a major operation, there's no point in tempting fate by poking around if your sensor is already pristine.

To test for dust and blemishes, set the camera to a small aperture, say around f/16-22, and take a picture of a plain white surface, such as a piece of plain white paper. Have a look at the resulting image on a computer and zoom in to hunt for tell-tale blemishes.

To make any blemishes show up clearly, adjust the levels of your images. This alters the contrast so dark bits show up even darker, making flaws easy to spot. Open your image editor's levels histogram and pull the left and right handles to the middle. This will boost the contrast and show up any imperfections.