best bet would be by the type of high you get....and for the most part Sativa buds are long and fluffy and Indica are more dense and small...

also if your guy knows the true name of the herb hes selling, you can look it up online... most seedbanks tell you what percentage of Sativa and Indica are in that strain... But for the most part bud is a mix of the two...ie 60/40, 80/20