S.S.
They must have liability insurance. Your homeowner's policy probally only pays $500 medical and it does NOT cover employees. I would verify with the insurance company myself.
I would ask for recent references. WHICH employees actually do the painting on your house? How experienced are they? You want to make sure they are not sloppy, leaving paint drippings on the baseboard or floor for instance.
Also, do they work until done or will they be leaving to give estimates and work on other jobs? You need them to come in and finish before starting another job unless the paint is drying.
Check the BBB website for complaints. I would ignore those with more than one complaint. It means they did not resolved problems until forced to.