This might sound really n00bish but I never though about it. Ok so you have a good niche but you want to check it out. You search it in google and it comes with with let's say 500,000k results. Obviously you need to know more then just the results. There could be 10,000 sites competing in the same niche or something. How do you find out the domains that are competing?
Well if you've figured out how to search on Google, then you have your answer. Just look at the domains that are appearing in the search results However, usually only the first 5 pages or so have actual competitors, the rest tend to be small sites that just mention the keyword and rank for some reason. In really small niches usually just the first or second page have actual competitors though.
A tool I find useful is "micro niche finder" the reason I like it is because it shows you strength of competition. This seems to be what you are looking for. If you dont want to pay for a tool then you could always use the wordtracker free keyword tool along with the google adword tool. A great way to find out how strong the competition is, is by looking to see who is on the first page of your term. If the first page is full of competitors like ezinearticles, youtube videos or other article based websites then it wont be hard to rank high. However if you find that your competitors are sites such as about.com, wikepidia etc then theres no way you can rank high for that term. Will Cooper