For Palaminos (up to 2100+), the best cores are AROIA/AGOIA Y. Supposedly week 13 is the best, able to do ~1.9GHz at default voltage. I have a AROGA Y week 4 1800+ that does 1.8GHz at default.
With Tbred A's, I imagine that all of them, from 1700+ to 2200+, should be able to do 1.9GHz or so, but probably not much more.
With Tbred B's, it's still too soon to tell, very few people have one of them.
How to tell when you go to the store... good question. I've had this issue before and the reality is that there is a lot of luck involved. If you get a Palamino from newegg.com, supposedly they only sell AROIA/AGOIA (and have great prices to boot 🙂 ). Sometimes they will also guarantee a particular stepping, but for extra $$. If you go to a brick&mortar store, I have specifically requested for a particular stepping and insisted on looking at the cpu before buying. Most stores are cool about this if you don't act like a jerk.
How to tell? Look at the core (or the little sticker on top of the packaging with Tbreds). The first line identifies the cpu (speed, etc). The 2nd line has the stepping and week of manufacture at the beginning, for example AROGA0204 or AGOIA0213 (stepping, year, week). The 3rd line is the cpu serial number. Here, look for a "Y" at the beginning, these tend to be better OC'ers.
Hope this was what you were looking for...