When can you tell the difference between varieties?

[cythaenopsis]

OK, I was pretty good for the most part, labeling my pots by variety. But... I wasn't perfect. I got a little lazy. And what happened was some seedlings didn't make it and I transplanted replacements into their pots (which worked great, btw), while forgetting to update the labels!

So at what point can you tell one seedling from another? I know with some it is obvious from the very beginning. When the Sara's Galapagos sprouted, the leaves were very small and ellipsoid, and they're growing rather slowly. However, the Russian Oxheart, Black Krim, and Black Plum all look about the same. The Russian Oxheart seems to have a slightly different foliage pattern, most of the time. But it's still close enough to get mixed up. So at what point should I be able to tell? Such as matching up known ones to unknown ones? The reason I'm asking is that I can't grow all of my seedlings and will have to give some away. I'd just like to avoid giving away unknowns.