As already written several times, the main tools for identifying keywords are:
Brain Storming , let's make the brain work: ours (SEO) and the client's
Competitor analysis (if we trust our competitors…!) based on content, positioning and backlink analysis
“Discovery” tools
For point 1 I'm sorry to disappoint you, there is no solution: probably a few benefits of using our homeowner database running sessions could be a good method to oxygenate the mind and generate new ideas, maybe together with the client.
For points 2 and 3 we say that SEMrush is one of the tools on the market that is able to offer a wide range of tools ( SEO Toolkit ) to support us in the search for new lemmas.
The available tools have been divided into two families: SEO Competitive Research and SEO Keyword Research .
Warning: don't turn off your brain, it will always be the one to decide what is good and what is bad!
One of the first rules of marketing (even non-digital) is that it is necessary to know the universe in which our business will have to live (market) and within that universe know the competitors .
In the digital world, the best solution would be to have access to competitors' analytics tools or their CRMs. Since I see this as a slightly utopian solution, the alternative is to use projection tools that can get closer and closer to real information.
For example, with Domain Overview it is possible to have a good overview of the competitor, at least from the point of view of its positioning in the world of digital promotion, both organic (SEO) and paid (Ads and Google Shopping).