I dug around out there a bit, on the Web I should say, and found some info to help those that will be hiring a steam cleaner to come in and do a thorough job on your carpets.
Check it out:
Angie’s List has an article which recaps the potential experience in a single page. It’s a nice outline of what a typical cleaner will do, what detergents they might use and what process they might take.
Oxyfresh has a page that talks about their process in brief. They mention that they go over everything twice, which is nice I guess.
Here is some more advice regarding considerations to take when hiring a cleaner. This one gets a little bit into pricing and what to think about in terms of cost.
It doesn’t hurt to ask a few questions either when you are checking around. You should ask about methods used to clean, what they use to clean (what types of detergents and machines) and also ask about the process. Don’t forget to ask about cleaning other upholstery you might have around the house as well. You might be able to get that cleaned along with the carpets.