Do not choose a contractor based on a single phone call. Instead, invite the contractor to your home or business, and ask for a free estimate; most quality contractors offer them. Free estimates are a great way to test the contractor's knowledge and to see his appearance and professionalism first hand, with no obligations. You will also have a written estimate, so there will not be any overcharges or surprise add-ons.
Do not choose a contractor that uses consumer grade equipment and cleaners. Cheap equipment may be a sign of a "fly-by-night" company. Big, high-dollar, skid-mounted pressure washers are not necessary to do a quality job. But, if the contractor uses the same little 2.2 gallon per minute pressure washer that you saw on sale at the home improvement store, you might want to reconsider hiring him. Most quality contractors use 3.5 to 5.5 gallon per minute cold water units for residential use. If a contractor is using consumer grade equipment, he is probably using consumer grade cleaners, also. House, deck, and concrete cleaners that are sold to consumers have to be safe to handle, therefore, they are not very concentrated or very effective. Contractor grade cleaners sold through distributors are still environmentally safe but are more concentrated and must be handled with care. Contractor grade cleaners are necessary in order to achieve quality results, without using high pressure, which could lead to damage.
Make sure proper insurance is in place. This means actually getting a certificate of insurance, and for larger jobs actually calling to verify actual insurance limits.
Check references; ask for photos of work done that is similar to your project, and how they did it.
When getting an estimate, ask for parameters. “What is the MOST that my project will cost?" If you don’t feel comfortable with the worst-case scenario, then perhaps you should make some more calls.
Have items removed from area/work space to be cleaned - Patio furniture, vehicles, toys, pets (unless they’re really dirty), etc. This will protect items from damage. Work space comprises more than just the area to be cleaned. Overspray, equipment, and hoses must be considered.
For larger projects: Sometimes commercial and larger residential projects require a walkthrough prior to the start of a job, to establish exactly what is to be cleaned. Prior to the beginning of work, see that the contractor has a written outline of expectations, so that it is clear what is expected, and there is no question about if/when the job is finished. Sometimes this outline is signed by all parties.
Establish date(s) and times that work will begin and an estimated completion date/time. This gives you, as the customer, the opportunity to be there for at least part of the work, so you can see that standards are met. This gives you the opportunity to be there at completion of work, and to authorize completion (or lack thereof).
Establish payment arrangements prior to the beginning of work, i.e. method (cash, check, e check, etc.), time frame (due on completion, net 15, etc.)