@Collin S. a word about Venmo... you'll probably have to issue your cleaning service/housekeeper a 1099 at the end of the year. Especially if you have multiple properties, it might be a good idea to use Quickbooks and pay them via bill pay so you can accurately track all of the expenses and not have to scroll through your venmo feed at the end of the year and spend hours tallying up the charges.
As far as what to ask, I've found effective communication is by far the most important part (for me) of a cleaning service. The bigger the company, the more layers of people you have to go through to get to the person actually cleaning your home. I would ask if you have someone to call/text or if email is the only option.
Obtain proof of insurance from the company or if you're hiring someone off craigslist make sure your insurance covers your liability if they sue you.
I've got some more specific guidelines for how I hire if you shoot me a PM I can go into more detail.