In general, watch the edges of terrain for game. By edges we mean anywhere different types of terrain meet, such as fence lines, timber lines, where C.R.P. meet cultivated fields, etc. Nature doesn't grow horizontally, so any straight, flat line could be the back of your quarry. Concentrate on ears, eyes, antlers, feet, etc. The chances of a trophy whitetail completely exposing itself is minimal.
Color is a dead giveaway.
A moose is proportionally about the size of a rabbit at 300 yards. Let your eye's natural ability to distinguish movement or abnormalities in the scene do the work for you. Use optical aids like binoculars to check out each hunch. Look for movement, especially in peripheral vision and while walking. If you are standing still, they probably are too, so be particularly aware of your surrounding when moving.