Yes, I've always candled eggs when I have bred snakes. But I handled the eggs gently, never turning them, keeping them in the same orientation at all times. No way I can tell you how long before an egg will appear to be non-viable: could be right away or go "full term+" and then either not hatch or be defective. So you're stuck with waiting ...even eggs that were never turned & candle reddish may keep us guessing.
As far as candling, some eggs take a while before they appear to be good, so I always wait for obvious signs of failure before discarding them- signs like oozing slime, moldy, smelling bad- typically draws flies, etc. Good eggs have a reddish glow from the blood vessels inside, & while 'bad eggs' appear straw-yellow inside, keep in mind that not all good eggs look reddish for a week or more after they're laid.
One more thing- most people prefer to stay on the safe side & never separate their eggs to incubate them. It's more work (tedious would be the right word!) to separate them but it's how I've usually done it, so that when one or more eggs go bad, they don't adversely affect the other good eggs they may be stuck to. As already mentioned, bad eggs draw flies which brings maggots which definitely don't differentiate between good & bad eggs.
But in order to separate a clump of eggs that are stuck together, you need PATIENCE: One at a time, I use my finger or an eye dropper to put a drop of water on the place where 2 of the eggs are stuck together. You can only work on those 2 at a time, ALWAYS keeping ALL the eggs oriented the same way. It's not easy, especially with a large clutch, but my hatch rates were just fine. Anyway, that drop of water softens the stuck part as you barely roll the eggs back & forth, until that spot loosens & lets go without tearing either egg. Then repeat the process until all eggs are free of each other. I mention this not because you necessarily want to try it, but just to let you know that "some handling" of the eggs will not harm them. But turning them causes the blood supply network to disconnect & then fail. I hope that makes sense?
