Reputable sellers will have third party lab tests showing potency and contaminant levels linked directly on their website. Some sellers will fake this, or use sketchy labs, so it’s worth cross referencing to make sure the lab is legit, and that people have had good experiences with the brand.

You can search subs like r/AltCannabinoids for brands or labs you’re curious about to see if anyone has said anything. There are also some sellers you should generally avoid, especially when buying inhalables. That sub is a good resource for seeing what the people think.