Is there a difference in the appearance of a sunrise and a sunset? 
For instance, if you're looking at a photo (which obviously wouldn't
tell you what direction you're looking in), would there be a visual
way to discern whether you're looking at a sunrise or a sunset? 
Thanks!!

In urban areas, sunsets may look different, since the air is typically
more polluted in the evening than in the morning.

"Let's think about what happens to the atmosphere during the day. It
warms up, which means it can hold more moisture, and traffic and other
activities churn a certain amount of dust and pollutants into it. All
this extra stuff in the air can make the sunset much redder than
sunrise. It can also make the light more diffuse. Artists sometimes
remark that the light at sunset is softer and warmer than the sharp
light at sunrise....

Another clue is the ground, which cools down overnight. This can cause
dew to condense out of the air, and mist to form in the air near the
ground. If there's any rising mist or dew in your photo, chances are
it's sunrise."

http://amos.indiana.edu/library/scripts/sunrise.html

I was a career Navy officer and spent eight years at sea, observing
countless gorgeous sunrises and sunsets, neither affected by
pollution. When I look at the pix I took of both, I, too, have trouble
determining whether I took them in the early morn or at night. Darn
good question you have asked!

It's funny that you asked this question, because I recently had the
same problem. I work at a stock photography agency, and had some
pictures of Alaska. It seemed to me that a lot of the pictures (for
keywording purposes) could have gone either way. Not all the vendor
information said if it was a sunrise or sunset. Some of the
sunsets/rises were very purple and I have no idea if Alaska
sunsets/rises are different :-) Sometimes it seems like a sunset has a
different glow than a sunrise though.