Gymnosperms (apart from Gnetophytes) don't have vessels in their xylem (wood); Angiosperms do. There are other differences, but it's a bit early in the morning for me to remember them all yet ;-)

Otherwise, it is fairly tricky, as convergent evolution has resulted in many characters developing similar appearance as an optimum for various purposes, even though in structural development and origin they differ (e.g. alder 'cones', yew 'berries', broad-leaved Agathis leaves, needle-like Erica leaves, etc., etc.).

Resin

The transporting tissue of Angiosperms are vessel elements. The Gymnosperms have a different structure, tracheids, that preform the same function. http://en.wikipedia.org/wiki/Tracheid

The presence of resin canals, on the conifers that usually have them (Dont remember which do and which dont), are a dead giveaway.