Many, if not most, hospitals are accredited through Joint Commission on Accreditation of Healthcare Organizations (JCAHO). This is a voluntary accreditation but it is a powerful tool in assuring that healthcare is delivered properly. Here is there web site:
Call the hospital in question and ask which accreditation organizations the hospital uses or go to the hospital and look in the lobby for postings.
You can check the JACHO site for your hospital's survey results. You can submit a complaint via the website, too. If you will, read some of the survey results before you submit your complaint so that you can determine which category best matches your complaint.
Your state's Department of Health and Human Services is probably the resource for review for Medicaid and Medicare services. You should be able to find your state's department on the web, also.
If you write a letter to the hospital's administrator, the best way to prevent your letter from being discarded is to include a statement that you have filed a complaint with JACHO and ask that your complaint be investigated during their next on-site review. The same applies to any letter you may send to your state's licensing boards for physicians, RNs, LPNs, etc.
If it is a surgical matter, be sure to send a letter to the hospital's Chief of Surgery. If it is a physician matter, send a letter to the hospital's Chief of Staff; if it is a nursing matter, send a letter to the Director of Nursing. For any concern, send a letter to the Quality Assurance Director (This title varies by institution; it may be Utilization Review or a similar title.)
Without knowing the nature of your complaint, I have tried to answer your question. If I have not met your specific need, please post again.