When it comes to any safety/crime related issues, it's always best to contact the local police department with all your questions, hear all there is to hear firsthand. If unfamiliar with the area(s) do revisit more than once and at different times of day, possibly chat with locals/neighbors. Real estate professionals are prohibited from steering, enticing a buyer to purchase/rent, or not, in specific neighborhoods. http://www.neighborhoodscout.com/ https://www.crimereports.com/ http://www.homefair.com/
Harlem is definitely changing, contact the local precinct in that area to find out the crime stats and trends, also as the previous agents mentioned its always best to visit the neighborhood at different times of the day as well as weekends vs weekdays to get a feel of the neighborhood.
On the issue of how "safe" a neighborhood is, we all have our own definitions. My suggestions are twofold. Visit the neighborhood at various times of day and night and see how you feel and what you observe. Secondly, contact the local precinct and inquire about crime stats.