Tree-of-heaven & sumac growing right next to each other on the side of the road, so I did a comparison study of these two easily-confused tree (primarily of the leaves). TOH has slightly wider leaflets with auricles at the base, leaflet edges not revolute, underside of leaflets light green, rachis glabrous, & petiole attaching below axillary buds. No milky sap, but crushed leaves & snapped twigs have a distinctive odor. TOH had already gone to fruit, while the sumac was still flowering.

TOH on the right/bottom in leaf comparison photos. See also http://www.inaturalist.org/observations/651253.