Suppose i have invented something useful. before applying for a patent is there any way which will help me in identifying whether the invention is already patented or not.
Patentability is determined based on whether your idea is new and non-obvious over prior art. Prior art does includes patents and published patent applications, but can also include non-patent literature such as webpages, journal articles, physical products, catalogs, photographs, and the like. Accordingly, determining patentability involves a search of all prior art, not a determination of whether the invention is already patented. Stated another way, an invention that is not be patented, or was never patented, might still not be patentable because it is not new and non-obvious in light of available prior art.
On the other hand, something is "patented" when there is a valid and enforceable issued patent that covers the given invention.
To do a prior art search, a good place to start is Google Patents. Also consider doing a general internet search.