Just look for things people would throw money at. This can be anectodal or with data.
Things with a high CPC are good targets. Things that have a lot of users but are built terribly or neglected are also good.
You talk to people. Friends or friends of friends. No formal research. Then you try to reach similar people and learn more on the go.
It takes time but if you succeed you both know your customers and a way to reach them
check, if the product/service solves my purpose
quick search for users# like me (profession or personality)