I use this > http://testico.net/ then i test it on my device (iphone 4S) where i relocate it in a couple of pages. I also put it inside a folder too to check if the icon is 'readable' there. Then i ask a few people if it stands out. Then sometimes, i sleep then check it first thing in the morning where i am groggy. If it passes all those then its a good icon. I may have to add a check when I am in a bad mood or hungry for my method to be ultra comprehensive.

If you have budget to spare, A/B test a few different variants on the Facebook ad platform.