I look for parks nearby and then take a look at them in satellite view on Google maps to see if they seem reasonable. But beware that many parks are in scenic areas....that are scenic for their hills, and hills don't really show themselves on satellite view.
If there isn't a park nearby, I'll tend to find a major road (usually if I'm traveling, it's in a city) and just do an out-and-back down the road to see what's there. The idea of being aware of unsafe neighborhoods is good but in this case, I'm generally just starting from my hotel, and hopefully my hotel isn't in an unsafe area.