Saw this first-hand at Nats this year. A friend got top 16 and won some packs, we weighed them all electronically; one pack was 2 grams heavier, contained a Klingklang. One pack had the Secret Rare Pikachu, 2 micrograms heavier. Similar story with the Ultra Reshiram. Weighing packs definitely work, but you need to be super precise about your numbers to know exactly what is lurking where.