Mike has a great idea (time lapse photos).
Also, the following site has great information throughout the world. It can't take into account trees, but it will tell you all else.
PVWatts Calculator
As for trees, I would go up on the roof and map out the parahelion (path of the sun) on the shortest day of the year, the respective equinoxes and on the longest day. Deciduous trees have leaves that fall in winter and they should not shade a lot.
How close are the trees to the roof? And how tall?
Steve