With difficulty! J. scopulorum tends to have bluer-toned foliage and J. virginiana greener-toned, but they overlap. The cones ("berries") if present are a better help, larger (6-9 mm) in J. scopulorum and smaller (4-6 mm) in J. virginiana; they also differ in maturation, with J. scopulorum cones maturing in 18-20 months from pollination (early spring to autumn a year later), and J. virginiana in 6-8 months (early spring to autumn the same year).
Here the dry-climate J. scopulorum tends to become ratty in cultivation whereas J. virginiana tends to maintain an attractive appearance.
there may be a difference in size: J. virginiana will get taller (65 x 14) and wider than J. virginiana (49 x various widths depending on cv.). Does someone know about the bark?
Does someone know about the bark? J. scopulorum: plates J. virginiana: strips http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=116875
Got a name doubled there by mistake ;-) J. virginiana is the taller, reported to 30m, and J. scopulorum the smaller, to 20m. But those figures are for mature specimens in their native habitats; it won't help much with cultivated plants.
