What elements on the map help to indentify Mexico and Canada as two countries that border the US?
In general, only maps that are considered "political" will show a definite dileniation of country or political divisions.
Color. The most common way that maps show that one country is different from another is with color. There is no one accepted color for the U.S.A. or Canada or Mexico. It depends on the map-maker as to which color the company assigns, but the colors will always be high-contrast colors so that one may easily see the difference.
Bold Type. Another way to tell that the area on a map is different from another is that each differently-colored country is clearly labeled in bold type.
Solid and Bold Dividing Line. A third common way to tell when you have moved from one country to another is with a solid bold line. This line will be darker than the lighter broken or dotted lines that separate states or counties within a country and will be unbroken.
Most maps attempt to distinguish different countries in two specific ways: First, you will usually find a line that differentiates adjoining countries; it is often a line with a series of slashes (and sometimes dots) but may also be a solid black line. Sometimes the dividing line is slightly bolder than others that separates states or other districts. Second, most maps use different colors to distinguish adjoining countries. For example, if the USA is colored in blue, then Canada may be shaded red, and Mexico green. Bodies of water, such as rivers and large lakes, often separate international boundaries. Several of the Great Lakes, for example, serve as a border between the USA and Canada. Mountain ranges and deserts are other geographical divisions for some international borders.