DockIconChecker is tool for OS X application developer.
This application can preview application icon on your dock without making .icns file and building in dummy application for test.

You can preview icon with JPEG, PNG, TIFF, PSD, EPS, GIF, PDF, .sketch, .pxm etc.
A basic operation is only drag and drop.
So it's possible to compare to examine multiple icons so quickly.
Also you can select icon with the arrow key and numeric key. This feature is very useful.

*Lite version allows you to compare the 2 icons.