There's a number on the bottom of each booster (20310760000001 for the last set for example) which ends in a 1, 2 or 3. I've found there's typically 4 boosters for each number. The only time I've not got the expected frequency is one case where there was a 5/4/3 split.
I tend to order through my FLGS, who have in the past let me open the case in store and check the boosters within.
