Go through the United States Patent and Trademark Office Website. They regulate the application process before granting patents. All claims are handled by the department and due to their international collaboration - links with the European Patent Office and Japan Patent Office - they are a good source of finding historical patent data. The USPTO has issued a full text of patents since 1976.
Your search will be based on the patent number, classification, issue date, inventor name. You can further search for a patent by visiting their office, located in Northern Virginia. The timing runs from 8 to 8 from Monday till Friday. Place a call before you visit the office.
One can further perform a regular Google search if you do not have authoritative access to the USPTO database. Free Patents Online is a similar search option, which gives you easy route to global patents including US, Europe, Japan (Abstracts), WIPO and German (beta version). It gives an expert search option for those who need to specify results. For the rest there is a quick search option, which then redirects the user to a new page asking them to insert relevant information in the given fields.
Google Patents also attempts to aid people with historical patents, going as back as 1790. However, it may not be a fool-proof way of gauging the authenticity of the document due to certain limitation of the software.
There are various online libraries that can help with your search. These include the National Archives, the British Library, or the specifically designated Patent & Trademark Depository Libraries, specified on the USPTO website.
Despite these free tools, one can find proper documenting by hiring the services of a patent attorney or agent. Even if you don’t intend to file for a patent in the near future, the essentials of patent history can only be thoroughly known through a professional who is qualified.
However, make sure that you hire an attorney who is trustworthy as you will be exposing your research to him. Contact the USPTO to find a licensed attorney.