Mars and Earth both rotate around the sun, but at different speeds. Their location relative to each other is constantly changing, as demonstrated by simulations presented by Windows to the Universe. In order to determine when Mars can be seen, the viewing location on Earth and the date must be known.
Maps showing the location of Mars relative to other constellations are widely available. The map provided by Naked Eye Planets shows the course of Mars across the constellations for a date range selected by the user.
A variety of apps and online calculators return information about where and when to look to see Mars. The programs ask the user to enter the location, date and time. The program provides a direction to look in and an angle to look upwards above the horizon to spot the planet at a particular time of the night.
Astronomical organizations post alerts and news stories about particularly good Mars viewing opportunities. For example, EarthSky.org notes that Mars is easily visible throughout July 2014, appearing over the southwest horizon just as the sun sets and remaining visible until midnight. For easier watching, the site posts additional date-specific viewing tips, such as an indication that the moon is be directly above Mars on July 1, 2014.