Solar Panels

I would like to get solar panels for my house which I own. I contacted a company and the salesman told us that it's not worth it because we have a partially shaded roof. That was the end of it. We may be able to cut some of the trees around our house but not all of them as they are not on our property. There is still quite a bit of Sun on our roof and we the roof has both North and east facing sections where we think we could potentially put the solar panels. I was thinking that even if the roof is partially shaded that of there is enough sun or may still be worth getting the solar panels. Is there some way to measure the potential efficiency of the solar panels? Are there solar panels companies that would measure this for us?