Answer: It takes a little bit of investigation and some guesswork. Look at the tuber, one end should have a little more 'hairiness' to it. Those are reminants of old roots and that is the end that should be planted down. Also, one end may be slightly more tapered/pointy. That end goes up. If you really can't tell, plant the bulb sideways--the roots will find their way down, the shoots up.
Hope this clarifies things for you!
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