Estimated reading time: 1 minute, 3 seconds
Whatever idea you have and believe can provide VALUE to others go check about it on the website (e.g Udemy) where you want to start selling online course.
If a course exists on platform then check its title and contents.
If you believe that you can bring better than the course, go make yours.
If you see the topic is in demand, then it is clear there is market to earn in that topic, just bring your own contents in your own flavor.
If a course does not exist, then Google on your topic and see if related audience (people) are talking about it anywhere and are in problem or not? Further, you may join your field related groups on Facebook and see what other people are talking about regarding their problems.
If they are in problem, then read their problems and craft your course to cover the problems.
At the end what matters is your own belief. If you believe that your course can seriously IMPROVE the student on the topic, then go for it but If you don't have faith in your contents, then don't go for it.
It is all my opinion, you may further research Google and decide yourself the best for your situation and subject of interest.