It is imperative that you know that they are insured and you should ask for a certificate of insurance naming you additional insured.  It will not cost the contractor a penny.  If the contractor did not have insurance, you could be held liable for 1) any damage done to YOUR property by said contractor 2) any damage done to other parties property 3) liability resulting from injuries caused by contractor 4) workers compensation due to contractors employees injured while performing their duties on your property.