How can we find out which products in the market are using a particular patent?

If your concern is freedom to operate, you are not concerned with "a patent" but with all patents which might be infringed by your product. A freedom-to-operate search and analysis would be appropriate. If you are concerned about IP associated with a particular competing product, and seek to find out what relevant patents are held by the competitor, you need to find relevant patents held by or assigned to the competitor and relevant patents licensed to the competitor. Often this entails searching advertisements, magazine articles and other statements by the competitor in addition to patent searching. Sometimes there is a press release that mentions a specific relevant patent. Access to a set of databases covering newspapers, magazines, industry publications, press releases, etc., retrospectively, is very useful. Also, many patented products have a notice printed on the packaging or the article stating the relevant patent numbers or application numbers. Often I have searched PR Newswire, Business Dateline, Chemical Business NewsBase, and other sources. As far as drugs are concerned, the FDA (online) Orange Book is also a useful resource.