Niche testing ... a few extra thoughts
There are a few things you can do to narrow it down.
First, be sure that it IS a niche. "Dating" is a huge market, for example, but it's much too big to be called a niche. Over 50s dating in Southern California ... that's more like a niche!
Second, look at it from the other end: is it so small that there'll be no meaningful market for it? Breeding toy poodles might be a niche, but "toy poodle's toenail clippings" would just be for a handful of weirdos!
Third ... does the market actually want to spend money in the niche? "Getting over heartbreak" is a moderate sized niche, but most people are looking for free poems or pictures of broken hearts. They don't want to buy a book or a course, for example. But "how to get your ex back" is a red hot commercial niche!
How to test? Easy! Google will tell you - and all for free.
First, go to the big G and type in your search term just as you would if you were a regular punter. Is the first page mostly Wikipedia entries, blog posts and free articles? If they are, then the sellers aren't working too hard to get their sites onto page 1 for that topic ... and that's often a clue. (Not definitive - you're only information gathering at this stage).
And how many ads are there on the right hand side? If those ads are still unique three pages deep, you've probably got something with some commercial value.
Okay, now check from the other end. Go to Google's free keyword tool, (just search for it on Google) and type in your keyword again. Google will not only show you 100 related searches, (800 if you have an Adwords account but don't worry about that for now), but there'll also be a little green or white or green and white box beside each entry. The more green it is the more competition there is for PPC - which means people are spending money on that search term - and so there's money in it somewhere too!
That's still only the tip of the iceberg, but it's a darn site more scientific than thinking a bunch of people must be interested in it because you are! (That's how I used to do it and it cost me a fortune - and a lot of wasted time too).
Hope that helps - and good luck.
Trev.