I prefer to use my nose and inspect the plant material before I buy it. Lots of strains are named because of the mother's that have been crossed to grow it. Once you know what a properly grown diesel, a kush and a haze should smell like, you can't go too far wrong. I try to buy the strains that smell so good to me, that I want to roll around in it. The smell is related to both the flavinoids and the terpenes in the plant. If these are right, even a lower % of THC will still produce an efficient high. Other things that can make a difference is the other foods that you have eaten about 45 minutes before you use cannabis. Ginger, chili pepper, hops, basil, mango, lemongrass, hemp seed oil and chamomile will prime the receptors in advance. This makes your high, higher and is referred to as the entourage effect.