Good eggs: whiter, drier, leathery, rounder, often stuck together (don't separate!!)
Slugs: yellower, wetter, rubbery, more oblong, never stuck together.
If in doubt, incubate. Good eggs can have white snowflakes, or be partially calcified, or have clear windows. You may not be able to tell a fertile egg by candling.
If you have a kingsnake, and are positive the eggs are infertile, slugs make a good snack!