Its a corm Marian, the bottom is rounded and the top slightly dished, don't bury it completly in the soil

I agree with Pam to a point, but sometimes it is not that simple, so to that end, you will find another couple of ideas here;

http://www.thegardenersalmanac.co.uk/Data/Begonia/Begonia.htm

If all else fails try next year storing them on a flat tray while you can see they all the right way up.

You can always plant them on their side because the roots will always grow down and the shoot will always grow up