The most accurate way of determining if a plant is indica or sativa is by appearance.
Sativa plants are tall and loosely branched and have long, narrow leaves. They are usually grown outdoors and can reach heights of up to 20 feet.
Indica plants are short, densely branched and have wider leaves. They are better suited for growing indoors.
Hillig, K. W. and Mahlberg, P. G. (2004), A chemotaxonomic analysis of cannabinoid variation in Cannabis (Cannabaceae). American Journal of Botany, 91: 966-975. doi: 10.3732/ajb.91.6.966