What tools do solar panel installers in Europe and Japan use for
measuring the amount of sunlight that will hit a particular location? 

There are ways to manually plot the skyline of a solar panel
installation site.  For example see
http://solardat.uoregon.edu/AboutSunCharts.html for a description and
http://solardat.uoregon.edu/SunChartProgram.html for automatically
generated sun charts based on locale. These methods are very tedious.

In the US most installers use the Solar Pathfinder:
http://www.solarpathfinder.com/
This device doesn't appear to be popular outside of the US,
however, solar panels are very popular.

The two large solar power plants being constructed in Germany are both
fixed installations of flat solar panels, tilted at the "optimum angle
of 28.5°" to the south.  This is a little more than have the latitude
of the installations.
Thus it seems that the designers chose to avoid the complications and
expense of having panels that follow the sun.

http://www.geosol-berlin.de/

http://www.br-online.de/politik-wirtschaft/mittagsmagazin/dynamisch/2004/09/20040908132700.htm
 
Hope that helps, Myoarin

Thanks for your comment.  I'm actually not interested in whether or
how panels TRACK the sun.  I'm interested in how installers choose the
optimum location when there are trees and houses that cause shading. 
If you look at the Solar Pathfinder it provides this kind of
information.
Willard

Sorry, I misunderstood what the sites with the calculations were for. 
Those large  installations are, of course, where no shadows fall, and
most small private installations here are on roofs, about the same
situation.
Anyway ...