Below is a nice example of 1936 5c Near Date and Far Date coins. The Far date has a narrow rim that meets the surface of the coin at a right angle, whereas the Near Date example has a wide rim that is filled - refered to as a concave rim. Note that for each coin, the rim is the same size for the full circumference.
Nice example of 1936 5c Near Date and Far Date coins
The best way for me to find a near/far date is to compare two coins against each other. When this is done, the difference is obvious. If you have a coin just by itself, it can be difficult to determine what it is. In the case of the images of the 1936's I posted, the thickness of the rim is an important factor.