Well there are several factors to consider. For one, the product could be relatively new and so not much affiliates are promoting it. On the other hand, the product owner could be making tons of sales.. you never know.

The only way for the product to have high gravity is if lots of affiliates are promoting and making sales. But just maybe the product owner sucks at recruiting affiliates.

My advice, is that if you have done proper market research and found it is a viable niche then give it a shot. It's better to try and fail rather that say >> If only I had done it!

Well, yeah..the best way to tell is to try it. Just spend $50 on Adwords for a quick analysis on the conversion rate. Else, you can do it with article marketing which requires more time and effort.

But once you see how it's converting, you can make better decision whether to pursue further or to drop the niche.

Else, a general indication would be to look at the no. of ads appear when you type in the keywords. The more ads, the higher the possibility of buying power in that niche. It's just a general assumption though.