Ask whether the people cleaning your house are employees or subcontractors, and whether they are screened. Ask for proof of insurance and bonding rather than a claim of it. Ask whether you get the same crew each visit or whoever is free. Ask what happens if something is missed, and who you call.
Ask what is not included, too, because that is where most disappointment comes from. Dishes and laundry are outside a standard clean at most companies, ours included, and the rest of the common questions are worth reading before you book.
Finally, ask how the price was worked out. A company that quotes off square footage alone without asking about bathrooms, pets or the current condition of the house is guessing, and a guess tends to get corrected upward once they are standing in your kitchen.