For your idea to be regarded as an invention, at least one significant part of its technology must be completely novel (that is, new). There must be no evidence that this novel aspect of your idea has ever been described before, or used for the same purpose before.
Not all the technology of an invention needs to be novel.
An idea may be an invention if existing technologies are combined in a way that is novel, or used in a way that is novel.
The inventive element might be only a small part of the whole idea. But if that small part makes a big difference to the commercial prospects of the idea, it could be an important and valuable invention.
Many people claim to have thought of a novel use of technology. The reality is that in most cases, the idea is already known. It therefore cannot be novel and so there may be little point trying to commercialise it. The inventor will usually be unable to protect it strongly, and without strong intellectual property protection to attract them (Part 5), few companies or investors will be interested.
How do you find out if your idea is novel? You do it by searching for prior art.