Another thing to consider is the search volume. This is the number of people who are searching for your particular niche. If the search volume is low, it's likely that the niche isn't very profitable.
Lastly, you'll want to look at the cost per click. This is how much you'll pay each time someone clicks on an ad for your niche. If the cost per click is high, it's likely that the niche is more expensive and therefore less profitable.
Also, consider narrowing your niche. The list of niches above is pretty broad, so there will likely be lots of competition if you are trying to target very broad keyword searches. By drilling down into a sub niche, you will have less volume, but less competition, and your audience will be that much more targeted. You need to find the sweet spot in a niche or sub niche, and it isn't always easy.
Doing this will help you to create highly targeted content and also make writing sales copy easier as you are targeting a really specific audience.
So, if you find that a sub niche is generating a lot of traffic or revenue, then it's a good sign that it's profitable. However, if you don't find either of these things, then it's a good idea to move on to another niche. And that leads on to the final stage...