Stop words as commonly referred to in the SEO industry, or non searchable keywords are words you should not target is keywords, for example if you search google for the term how do i find a good deal on cars it will return the following (The following words and phrases are very common and were not included in your search: "how do i find"),  so to make sure you don't target keywords that are stop words and can not be searched through, all you have to do is go to the search engines you are targeting and search for those keywords ! if they are stop words the engine will tell you.
 

You should also note that if you search for a stop word alone in google, it stops acting like a stop word, so to identify stop words you should search for a term that contains the words you are checking