Right, you can't tell for sure until you open them up and check a disc, but you can make some educated guesses based on the country of origin.
There is usually somewhere on the spindle where it says "Made in xxxxxx"
You can't go by the company name or there corporate address, you have to find the "Made in xxx" which is often in very small print.
I always try to find blanks that are "Made in Japan" I'll settle for "Maid in Taiwan" if I have to (although CMC Mags are Taiwan made).
Avoid blanks made in Malaysia or China.