A preliminary U.S. patent search is ideally a classification search of granted patents and published patent applications conducted to determine patentability of an invention.
The United States and many other countries organize their patents by the Cooperative Patent Classification (CPC). Thus, a preliminary patent search is to determine which CPC class or classes one's ideas fall into - and then seach that CPC class(s) to find the patents in that class(s). A Classification search can be supplemented with a keyword search, though there are dangers with relying on a keyword search only.
Although the tutorial above shows how to do CPC classification searching for granted U.S. Patents in PatFT, one can also do limited searches of patent applications in AppFT. The AppFT search choices can be found by scrolling down the USPTO Patent Search page - and offers the same Quick Search methods as PatFT. One can search the same CPC schemes in AppFT that they searched in PatFT.
(Espacenet allows one to search for patent and patent applications of 90+ countries -- or one can search just U.S. patent and patent applications)