Narrow, widely spaced leaflets show up by the third or fourth node on a true sativa seedling, and the internodes are already longer than on an indica of the same age. That early look is a reasonable check if you suspect a pack was mislabelled or mixed. A side by side seedling series at two, three and four weeks is posted on https://hazesativaseeds.com and shows the difference more clearly than any written description. The first pair of leaves tells you nothing though, they all look alike.