I guess it depends on your keyword, for instance let’s say I wanted to target a site that had been sitting in position once nicely for the past 12 months, I could get some traffic information on his site assuming this was a site pretty much dedicated to the one keyword.
Also another way which sounds a bit crazy but get to position one in google and check in google analytics, this will give you the best results then just play around with them, shorten them etc,
It depends how much time you’re willing to put into that keyword if you’re really out to get it, just wait and see the data in analytics. Go for long tail keywords random keywords that not really many people would think of “just you”, do a good bit of SEO and you should be there in a few days / weeks then you got all the information you need, you can just swop and change them, I do it all the time, just do it and see what’s working best for you in analytics. Once you get a good bit of data it’s easy to pick the best keywords.
If you don’t want to use any of those ways try Google trends