How to validate the attractiveness of your mobile game/app icon?

Hello~

I've already made an iPhone game. I've prepared several versions of icons and hoping to test which icon is the best.

Any good (and cheap) method to test which icon does attract most users? Please advise.

I currently use Admob (the mobile advertising network), where I make advertising banners with icons and choose the icon with most click rate.

Any other ideas?