There are various devices on the market for measuring sunlight - the link below contains three or four different ones, although I can't speak for the accuracy of any of them. Although it's easy to judge that an open, south facing area with no surrounding buildings or tall trees will receive any sun that's going, within the average garden or planted area, it's not quite so easy to judge if you're not there all day, observing over time, so it's probably worth getting one if you really don't know, given the amount of sunlight an area receives can differ enormously throughout the year, depending on the height of the sun.
http://www.amazon.com/Luster-Leaf-1875-Rapitest-Calculator/dp/B002XZLLXU