Different amount of hairs, different amounts of crystals, different smells, different tastes, different textures, different colors.

A general guideline--Indicas are often darker green than sativas, and tend to have a smell ranging from fruity to citrusy even to chocolatey!

Sativas tend to be a much brighter green and smell anywhere from piney to flowery to earthy, and also overlap into the citrusy range.