How do you test if you have a good profitable affiliate product before you put too much time in and realise its a no go. For instance my affiliate review site has went down the toilet for want of a better word. Over the last 6 months it has went from improving with sales to flat nothing. Instead of wasting the last 6 months beating a dead horse how could I have just came to the conclusion enough is enough and moved on? Promote the product for a month and if traffic/sales aren't looking good move on? I came across this great case study http://www.top5seo.co.uk/how-i-grew-a-niche-affiliate-site/ and my site is based around the exact same method except its multi product focused. Mine peaked at his month 4 traffic wise then collapsed.
I typically look at the amount of promotional tools that a vendor gives me. I feel like i can sell anything just as long as i have the proper materials to do the job with. But nevertheless, if marketing tools aren't given, i just create them myself.