Anyone can put paint on a wall however, for a result that will continue to look good for years, it is the start that makes the finish.
Abraham Lincoln said “Give me six hours to chop down a tree and I will spend the first four sharpening the axe.”. Similarly a good decorator will spend as much time, if not more, preparing a surface as actually applying paint or wall paper.
A smooth, imperfection free surface that is prepared to ensure paint sticks to it will stay colorful and fresh looking for longer. When papering, it’s equally important to start with a properly prepared surface. This is partly to ensure the paper adheres to the wall but also because imperfections under the paper are going to show up as smooth paper sticks to and grips the wall.
In our opinion how a decorator prepares a surface is as good a way as any to tell a good one from a bad one. We'd recommend that you ask what preparation a decorator will do. One decorator may be significantly cheaper than another, but the reason may be because they are not going to spend time on preparation which, in the end will mean a lower quality job.