Its practicaly impossible to tell the strain, and honestly, strain isnt too important when you are just scoring to get high for the sake of feeling high. Species is more important, and very easy to tell. If the leaves are long and thin, almost daggerlike, it is most likely a SATIVA weed. THis means the effects are very heady and stoney. If the leaves are short and fat, almost like a maple leaf, then the strain is Indica. Indica tends to be a much moer body like high, and energizes the mind. Of couse, there aer hybrids that share both strains features, so you wont be able to tell 100% either way, until you smoke it. if the effects are strong on your body and you feel jazzed, its probably a strong indica...but if you feel the indica affects as well as some slight headiness and mind wandering, it might have a dash of sativa in it as well.
Hope this helped.
-PM