This article is from the CD-Recordable FAQ, by Andy McFadden (firstname.lastname@example.org) with numerous contributions by others.
There is one approach guaranteed to work: put the disc in the drive. If
it works, you have it right. If it doesn't, eject it and turn it over.
Alternate approach: many discs have numbers or letters printed near the
hub. If they appear to be written backwards, the disc is upside-down.
Another approach: hold the disc edge-on in front of you, so you're looking
right across the surface of the disc. Tilt it up slightly, and look
closely at the edge farthest from you. When it's the right way up you'll
just see the label, when it's the wrong way up you'll be able to see
some light through the polycarbonate.
Yet another approach: the area of the disc near the hub may feel different
(one side may have a groove or a lump that the other doesn't). Figure out
which side is which, then remember how the disc feels.