Many roofing companies claim to have insurance when asked. And they may have some form, but how insured are they? Do they have worker's compensation insurance? How about liability? The questions you ask are crucial. And even if they say "yes" to your queries, you should get a copy of their worker's compensation and liability insurance, just to be sure, because sometimes people will tell you what they know you want to hear. When you ask them for proof they'll tell you "I'll get back to you."
Also, a little known trick that many roofing companies exploit is to have insurance but sell the work out to a subcontractor who may not have insurance. If that subcontractor falls off the roof and breaks his leg or forgets to set the parking brake on the dump truck and it plows into your living room, you could be left with the bill.
Again, ask questions and demand paperwork. Your home is too important not to.