Do you mean the thumbnail, the "Look Inside" image, or the in-Kindle image?
For the latter two, you can get a fairly accurate idea by looking at it on your desktop or by porting the image to your Kindle if you have one. As for the thumbnail, good luck. I can't seem to consistently get nice sharp thumbnails of my covers unless I avoid use of color/shadow gradients and fancy fonts. I do, however, resize my files in-process on-screen to check readability of title and name and determine if the images are clear enough to be recognizable and not a gmish of colors or shapes. My cover thumbnails always look better on B&N and Kobo than Amazon, but most of my sales come from ammy. Take from that what you will.
The nice thing is, if you're unhappy with the cover/thumbnail, you can always tweak it.
