There is a book I recently purchased called "Never Pay The First Bill". It covers this exact kind of scenario and tells you what to do step by step, all the way up to small claims court, though you usually don't need to go that far. Highly recommended.

More than likely you're going to need to get an itemized bill from the hospital listing the actual icd-10 charge codes and amounts for each service. Then you will need to cross reference this with your insurance's Explanation of Benefits document, which shows what the insurer paid for each code and the remaining that you owe. If there are any codes missing, there has been a miscommunication between the hospital and insurance, and the insurance was not billed for something. It is also possible there will be errors in the codes themselves, charging you for things you did not receive or received less than they said (charge code for 1 hr complex appointment when your appointment was 10 minutes). In this case, you will need to request your medical records, and cross reference them with the charge codes to dispute the wrong codes.

It also tells you how to deal with collection agencies.