Look on the product or product packaging for a patent number. All reputable companies "mark" their products with the patent number(s) that cover their products because that way they can recover monetary damages should anyone infringe the patent. No marking, no monetary damages until the infringer is on notice of the alleged infringement.
You can also search the patent database that's maintained by the US Patent Office at http://patft.uspto.gov/netahtml/PTO/search-bool.html . Type in the name of the company that you want to search and identify it as the "Assignee name." All of its issued patents should be returned.
You can also contact the investor relations department of the company and simply ask them to send you a list of their issued patents.
There are other ways to go about gathering "competitive intelligence" about a company's patent portfolio and some companies provide nothing but that service. Query "'competitive intelligence' and patent" and you should find those companies.
The advice of my colleagues is a good start but but there are additional steps that you need to take to answer this question. Companies sometimes disagree with each other concerning whether a certain patent covers a product. The manufacturer might take the position that various patents do not cover its product, and therefore the manufacturer does not "mark" the products to to refer to these patents. A competitor of the manufacturer might disagree, asserting that the manufacturer's product infringes patents belonging to the competitor. Therefore, it usually is necessary to conduct a rather broad patent search to identify the patents that potentially cover a product.
Also, there are different categories of patents that you need to consider. There are "product" patents that cover the product, itself (or in the case of pharmaceuticals, its active ingredient). There are "process" patents that cover processes for manufacturing the product---(often times there are different ways to make a product, and different patents covering these different processes). There also are "methods of use" patents covering methods of using a product for a specific objective or to achieve a particular result. (For example, a patent might cover a method of using a drug at certain dosages to successfully treat humans suffering from a specified disease). When a client asks me to conduct a "patent clearance" for a proposed product, it ordinarily is necessary to consider each of these different categories of patents. Even where a company tries diligent about "marking" its products with identifying information about patents that it owns which cover its products, there may be other patents covering the product, processes of making it, or methods for using it and you need to take these other patents into account.
Unless you are an experienced intellectual property attorney, you should not be making judgments on your own about whether an item is patented. If you plan to manufacture a product that is already being made by someone else, it would be prudent to retain an intellectual property attorney to search, analyze and advise you concerning potentially applicable patents. While such patent analyses do not come "cheap", the money you pay intellectual property counsel to advise conduct a patent search and advise you on this question, will end up saving you from a boatload of trouble (and money) in the long run.