It is indeed difficult, if not impossible, to tell at times. Here's a list a strategies I might use to tell the difference:

• Look for contextual clues. Even a tiny recognizable feature could reveal the cardinal direction.
• Atmospheric clarity. During a sunrise, the dust has had time to settle at night, making the sky clearer than at sunset, where there is a lot of particulate matter. You might be able to find minute differences in this and, combined with other factors, make a determination.
• Tilt of the Earth. This requires that you have at least two photos from the same sequence but with enough time in between that the sun has had a chance to move significantly. In the northern hemisphere, the sun rises up and toward the right, while it sets down and toward the right (which is the opposite since if you interpreted this as a sunrise, the sun would appear to rise up and toward the left). The reverse is true for the southern hemisphere. This requires knowing which hemisphere you're in, and ideally far away from the Tropics of Cancer and Capricorn.

Source: http://www.livescience.com/34065-sunrise-sunset.html