Best thing i can tell you to do is buy a digital scale. They can be pretty expensive, probably starting at $50. I bought a nice one with a backlight so i could use it at night without turning on my lights in my car, cuz thats kinda suspicious, and that was $75. However, it has helped me split up bags with my friends and kept me from getting ripped off. Also, its so much easier to smoke for free, cuz you can buy a big bag and sell a couple grams to ur friends.