if its dried jus smoke it an see a sativa wont put you to sleep an indica will or may do
indicas a heavier stoned an in my experience in growing the buds tend to be a bit denser an broader leaved shorter plants
sativas tend to be a bit more leaffy an fluffy but it does really depend on the growing factors, ive had sativa plants with solid buds before an indicas that are fluffy.
as the other post by stinkyattic says it is near impossible to tell strains indica or sativa is possible but still very difficult without smoking it, mainly due to the fact that each plant will tend to grow differently and a sativa bud may look indica due to the growing factors involved(light,nutes,soil etc)
id jus reccomend smoking it an if its a up head high its more sativa an if you get lazy an couchy then an indica
most commercial weed will be indica or a hybrid indica/sativa, as sativas generally take around 12 weeks too flower an its too long for most commercial growers, plus the yeild tends to be less in most sativa strains
if the palnt is tall with slender leaves it will probably be sativa if its shorter with darker broader leaves its probably a indica but there are no garuntees