One of the basic requirements of patentability is- the invention has to be new and non-obvious.
How do you find out if your invention meets those criteria? You do a patent search.
A patentability search is basically looking through the records of issued patents, published patent applications and other relevant material to see if the invention is new. That body of relevant information is known as “prior art” in the patent field.
First, you can and should do an initial search online yourself. The best free source is probably Google Patents.
There is no legal requirement to do this before filing an application, but filing costs money and the Patent Office will do a thorough search for your invention. Doing a search beforehand can spare you the waste of time and money applying for a patent on an invention that already exists.
If you search and find the invention already exists, then you’ll have saved yourself the money of having a professional do a more thorough search. But even if you don’t find anything, having a patent attorney do it is still a good move, since they will know how to look in places that may be difficult for you to locate.
There are professional patent search firms; the idea being to cut out the attorney and do a thorough search for much cheaper. The problem is- they tend to look only where you tell them to look.
This is where the expertise of a good patent attorney will come in. The attorney would help see beyond the immediate invention, looking for modifications that may expand the scope, or even circumvent previously existing similar ideas. A good patent attorney may uncover useful avenues that a search firm would not.