Ask your potential cleaning company if they offer any satisfaction guarantees. Do they stand by the work they do?
Be sure to ask what kind of cleaning products they use and if they are safe for your household pets and your kids. Some companies will use your cleaning supplies if you ask them.
Find out if they bring their own equipment (vacuums, brooms, mops, etc.) or will they be using yours or asking you to go out and buy certain items.
Another important question to ask is, if you are not happy with a cleaning job they performed, will they come back out and re-clean at no cost to you? Most cleaning companies should also be open to suggestions, if you have a particular area you want extra clean.
Make sure your cleaning company has a main telephone number and office where you can contact someone if something should go wrong.
Definitely ask for a list of references. You need to be able to trust the company you are allowing into your home and around all your valuable items.