Even though sometimes bud can look and smell great but not be as potent as you might think, IN GENERAL, when you see bud that is clearly covered with trichomes/crystals, it's usually good bud. Don't confuse cocaine with trichomes! In some areas, dealers will throw small amounts of coke on the bud to give it the appearance of having more crystals or to deceive newbie smokers. Trichomes will be skinny with little round heads on the top, and should be a clearish amber color. Coke will just look like small small grains.

I wouldn't depend on smell to determine whether its good bud or not. Just look at the bud closely!