1. Interview a professional service and avoid hiring an individual if at all possible. Why? Three reasons: (A) Liability, (B) Reliability, and (C) Effectiveness.
Liability: Hiring an individual puts a lot of additional legal requirements and liabilities on homeowners. Really? YES! Hiring an individual means you are now Federally liable for collecting, reporting and paying taxes on your new 'employee'. In addition, 99% (or better) of individuals cleaning DO NOT carry proper general liability and worker's compensation insurance. They fall, get hurt, break things or damage surfaces in your home and you/your homeowner insurance is 100% liable. Would you really risk the largest investment in your life to save a few bucks?
Reliability: If an individual gets sick (or just has something better to do...), guess what? Your home doesn't get cleaned that day.
Effectiveness: if your home is more than 1,000 square feet, it is simply too much for one person to clean thoroughly with detail in one day. People get tired, things get skipped or not done well. Think about it. Could you do it (much less do 2-3 more in a day?). By the way, if they are that good and they're employable...they'll make more money with a Professional Service, so why aren't they working with one?
2. Know what you want cleaned. For the most part, you're paying for time. Even if a service has their own cleaning system, many services can offer customization to clean the critical areas you regularly use and allow you to just 'add on' the extra guest room or bathroom when you need to. Also, be reasonable. You are hiring people, not machines. They work hard and deserve to be treated with respect as any human should.
3. Ask about background checks. 3rd party referral websites are nice, but they almost never verify background checks. So ask: (1) What kind of background checks do you do? This is important because local background checks typically only cover the County the employee currently lives. Anything outside of that county WILL NOT SHOW UP! Do you want a felon from another County or State cleaning your home? A reputable service will run Local, National and a driving history check.
Follow up to ask what they'll allow to show up on a background for employees. Will they allow DUI's, drug offenses, misdemeanor shoplifting...? If yes, end the interview and say "NEXT". Again you're trusting your family and possessions to be in their hands.
4. Ask how employees get paid. Hourly is good. It means they are paid for the full amount of time in your home. Commission/percentage paid employees mean they have to fly through xx number of homes in a day to earn a decent paycheck. The same goes if they are paid by the home (would you get agree to get paid the same to clean a 1,000 sq ft home as a 4,000 sq ft home?). It's also good if they get paid for travel time and/or mileage.
5. Check online reviews and look at the service's website. Let these 'guide' you, but don't make a decision based on these alone. Some services pay for good reviews and anyone can make a web page look good these days. It's also nice to see if the company is involved in anything community-related.
6. Find out how long they've been in business, where they serve, how many customers they have and what products they use. This will tell you how stable they are, if they understand your area and if you will have to add the extra cost of providing the cleaning materials, or if the service does bring their own clean supplies, are they 'healthy' cleaning products. It also gives you the opportunity to ask for some local references. Be sure to check them.