Step 1 - Identify potential markets
Typically the inventor/innovator is an excellent source for this information because they invented the product because it could not be found on the open market (usually) or the invention is the product of research that had specific goals in mind, e.g. to develop a better battery. It is also important to brainstorm with others because different backgrounds and experiences may point to more uses than the inventor specifies.
If it is difficult to identify how or where the invention could be used, identifying similar products may give some clues to utility of the product. If the identified markets are obscure or the usefulness of the invention is not clear, a database search may indicate new markets or avenues of research.