From ask the Meatman - 8. Pear, Peach and Plum. These woods require a certain level of expertise in their use. Peach and Plum woods tend to lose their flavor shortly after being cut. For the best results, make sure you the fruit bearing kind of Plum. - http://www.askthemeatman.com/woods_to_use_for_smoking_art_updated_9300.htm
This is probably not going to help much. I went out into the yard and took photos of the peach trees and plum trees. I have both in the yard. I guess the peach looks more "barky". When I cut limbs from it the wood is orange color - like a peach. The small limbs on the plum is kinda smooth. They make great smoke wood.