The questions the title of the thread but, how would you know? Also does it cost a lot of money to got something patented?
The USPTO has information about patenting and cost (http://www.uspto.gov/main/patents.htm) and you can search the patent database for inventions that are similar to yours. Really, though if you think you have a patentable invention you should get it evaluated by a patent agent or patent attorney (see the USPTO link for a list). They can do a search through the US and foreign patent databases (and the relevant non-patent literature) with an expert's eye - the way patents are written it can be difficult for a layperson to understand what they cover. Will it cost $$$ - yes, unfortunately. If you have a valuable invention, it's worth it, though, and if you don't you aren't out the money you would have spent on trying to patent it. But keep in mind, even if your invention is patentable, it might not make you any money, since that's unpredictable. Whatever you decide, don't go with one of those invention promotion outfits that advertises on late night television or in online ads. Read the warning signs here: http://www.ftc.gov/bcp/conline/pubs/alerts/invnalrt.shtm