Question on thinning out my TPS seedlings

[gggeek]

So I posted earlier in the month that my TPS sprouted. Well, it really freaking sprouted. I had so many TPS that I just planted a bunch not expecting much to happen. Now I have a ton of over-crowded plants growing just starting to show their first true leaves.

Is there anything at this stage I can look at that may give me indications for overall health or keys to certain features? There are a couple that are showing some great vigor over the others.