First off, you want to make sure whatever it is you’re buying is thoroughly coated in trichomes and all of those trichomes still have their glandular trichome heads connected and intact. This will tell you that the cannabis has been treated well since it was harvested.
Next, you want to make sure you’re not paying over the odds for a bunch of leaf. See that the bud has been properly trimmed. Most of the sugar leaf should have been removed, and you should be left with tight, trichome rich calyxes which make up the buds.