It used to be possible to do this with Google Trends, but they removed the number of searches and the y-axis is along the lines of 100% popularity now.
Otherwise I just compare to competitor products to see if the market is sizeable. I've done analysis before with a cofounder and things like keyword popularity also helps, but in the end it doesn't really do knowing that there's a huge market for "jobs for teens" sites unless you want to make a product for that.
