One way is to think about instances in your work or life where you've encountered a problem, searched for a solution, and settled on a solution that either took a lot of work or was suboptimal (or, better yet, didn't find a solution at all). If you've encountered that problem, there's a good chance that others have, too. Once you have some ideas for products, you can validate them by checking Google Trends to check if people are searching for it, or by creating landing pages for the products (that don't exist yet) to measure inbound traffic.
Another approach is what Paul Graham said: "Live in the future and then build what's missing". You can start working in a leading-edge area of technology and build things that are missing. Others that are also working the field may be searching for a solution to the same problem and then find your solution.
In any case, it will probably take multiple tries or pivots before you find product market fit.