Ask for an estimate and when they come out look at their equipment does it look neat is it a truck mount unit or a little hand unit? If it is not a big truck mount you probably will not get a good job. Ask them to explain the steps and take notes this will help keep them honest when they do the job.
Be careful looking for a budget
Manny's Carpet Cleaning Service you usually get what you pay for. If they seem to charge a little more chances are they will do a better job. When they come out to clean the carpets observe them that they do what they said they would do but do not watch their every move this will probably make them nervous and you may not get as good of cleaning job, think of how you feel when someone is constantly watching you work. Good luck on finding a professional carpet cleaning service.